Adding Sustainability in Analytical Chemistry Education through Monitoring Aquarium Water Quality

: This paper introduces a captivating topic for upper-level analytical chemistry capstone projects, focusing on aquarium water analysis. This provides a more comprehensive understanding of the role of analytical chemistry towards sustainability and its environmental, economic, societal and education dimensions. Regarding the crucial role of maintaining optimal aquarium water quality for the welfare of aquatic life, students are tasked with envisioning and executing the measurement of key parameters, including pH, ammonium, nitrite, and nitrate contents. This hands-on experience not only engages students in real-world applications, but also allows them to delve into essential analytical chemistry principles. They carefully select measurement methods, considering factors such as instrument availability, ease of use, precision and sensitivity requirements, sample size, and matrix effects. Besides fostering the acquisition of technical and soft skills, one notable aspect of this type of project is the exceptionally high student satisfaction. Furthermore, the project’s outcomes have proven to be signiﬁcant predictors of learning achievements. Additionally, it lays the foundation for exploring potential designs of aquaponics systems and fosters interdisciplinary projects, expanding the practical applications in the ﬁeld of chemistry education. Overall, these projects exemplify enriching and engaging educational experiences that empower students with valuable skills and knowledge while encouraging them to explore novel avenues in analytical chemistry.


Introduction
University-level chemical education to a great extent focuses on disciplinary skill training, content coverage, and preparation for final examinations [1]. However, this is not incompatible with using real-world examples to illustrate the relevance of chemistry to students' everyday lives [2]. Actually, exposing students to real-world chemistry may illustrate chemical concepts and principles while providing connections between everyday life issues and the topics facilitated in the classroom. Students often express dissatisfaction with the absence of the aforementioned connections in their educational experiences. Thus, introducing an interdisciplinary topic as a proposal could lead to more favorable educational outcomes compared to traditional laboratory exercises [3,4]. Notably, there has been a growing interest in interdisciplinary higher education over the years, as a cross-disciplinary approach enables a more comprehensive understanding [5,6]. In line with this idea, one can find in the literature interesting undergraduate interdisciplinary projects based on water chemistry and the use of water samples from various sites along a river, lakes, surface water and groundwater in urban areas, seawater, sewage water, swimming pool water or just drinking water [7][8][9][10][11][12][13][14][15]. Clearly, the context of water presents a valuable opportunity to engage students in analytical methods and foster environmental awareness, shaping them into responsible citizens [16]. For instance, certain activities can be focused on addressing be focused on addressing the negative impact of climate change [12]. Furthermore, implementing service-learning projects adds an enriching dimension to education, encouraging students to reflect on the broader purpose and significance of their learning process and its potential societal impact [11]. For secondary and high school students, there are also intriguing examples, such as participation in international citizen science projects dedicated to monitoring water quality [16][17][18]. These projects not only offer valuable learning experiences, but also promote the development of an environmentally literate society, by bridging connections between environmental education and other disciplines like chemistry [19]. Notwithstanding, few project-based learning works have focused on the use of aquariums as simplified ecosystem models and exceptional natural environments for laboratory practices in which chemistry skills can be developed [20][21][22][23][24]. These published works deal with seawater aquarium in the context of general chemistry or chemical engineering teaching. Specifically, the program "Chemistry in an Aquarium" proposed marine aquariums for the interdisciplinary teaching of chemistry [23], while Keaffaber et al. designed a marine aquarium as a model system for a general chemistry class [24]. Nonetheless, freshwater aquariums can serve as a viable alternative to saltwater aquariums or natural bodies of freshwater, particularly when incorporated into an aquaponics systema dynamic combination of fish and plant production within a recirculating environment, see Figure 1. Although saline aquaponics is being considered by some authors, very little research on this topic has been explored and freshwater aquaponics is the most widely accepted aquaponics technique [25]. From an educational point of view, aquaponics based on freshwater aquariums (as an aquaculture unit) might provide an excellent tool with which to integrate several disciplines and promote the training of systemic competencies [26]. Systems thinking is a promising way to enhance chemistry education [27]. For instance, students may explore biology via observations of animal and plant life cycles, investigate chemistry while analyzing water quality and apply basic engineering to calculate water flow rates [28]. What is more, the connection with living plants and animals exposes students to the natural processes of ecosystems [28]. Therefore, freshwater aquariums, rather than marine aquariums, enable the inclusion of more complex tasks that require interdisciplinary treatment, From an educational point of view, aquaponics based on freshwater aquariums (as an aquaculture unit) might provide an excellent tool with which to integrate several disciplines and promote the training of systemic competencies [26]. Systems thinking is a promising way to enhance chemistry education [27]. For instance, students may explore biology via observations of animal and plant life cycles, investigate chemistry while analyzing water quality and apply basic engineering to calculate water flow rates [28]. What is more, the connection with living plants and animals exposes students to the natural processes of ecosystems [28]. Therefore, freshwater aquariums, rather than marine aquariums, enable the inclusion of more complex tasks that require interdisciplinary treatment, in case of expanding the work to the design of aquaponic systems. Consequently, the employment of a freshwater aquarium as a teaching tool for analytical chemistry could be an original and gripping example of the integration of real-world elements into a chemistry lab classroom. Specifically, revolving around the aquarium water quality, given that they are essential to ensure the living conditions of the fish and the plants, analytical parameters i.e., pH and the concentration of some ionic species, should be monitored periodically to maintain a healthy ecosystem and an appropriate balance between the fish waste generation and the plants' nutrients [29]. Moreover, water is the medium via which all needful macro and micronutrients are transported to plants, as well as the medium thorough which fish receive oxygen.
Colorimetric analyses are commonly employed for water monitoring by using analytical methods and/or commercial testing kits. Amid the selection of suitable methods for the determination of the main water parameters, the evaluation and interpretation of the results from the experimental analyses are critical for the detection of uncontrolled concentration levels, as well as the establishment of corrective actions and the reconditioning of the water quality to keep the aquarium system working properly.
Pedagogically, three key strategies are suggested to promote meaningful chemistry education: incorporating relevant contexts that demonstrate real-world applications, presenting content on a need-to-know basis to focus on essential knowledge, and fostering an environment where students feel that their input is valued [30]. Hence, the use of an aquarium is a challenging framework for teaching analytical chemistry, allowing the principles and experimental procedures related to classical and instrumental analytical methods to be introduced and general topics such as stoichiometry, solution concentrations, unit conversions, redox reactions, acid-base chemistry, and complex formation equilibria to be reviewed. In addition, aquarium-based lab activities offer the perfect blend of exploring an intriguing aquatic-life system while simultaneously applying the corresponding analytical methodologies, including both classical and instrumental methods in the laboratory curriculum. This type of instruction could provide a more realistic image of analytical chemistry according to its fundamental role in the scientific field and its applicability in today's society; it may furthermore provide an attractive and dynamic resource that increases the interest of students, as the water quality of the aquarium is essential to ensure the living conditions of the fish and the plants that are in it [31][32][33].

Final Degree Project Setting
The study program of the BS Chemistry Degree at the Complutense University of Madrid (Spain) includes a final dissertation research project, a BS thesis (450 h), in the last year (usually the fourth). Every department of the Faculty of Chemistry generally offers different topics under the corresponding teacher supervision. During the 2019/2020 and 2020/2021 academic years, the Analytical Chemistry Department offered the topic "Aquarium water quality monitoring" based on the setup of a freshwater aquarium, and its water quality monitoring. Since BS thesis students must individually perform their final project, the proposed teaching-learning experience was carried out with one student for each academic year, although both students were supervised by the same tutor. These students had completed three compulsory subjects before starting their final research project: Analytical Chemistry I (225 h) in the second year dealing with classical (gravimetry and titration) analytical methods; and Analytical Chemistry II (225 h) and Analytical Chemistry III (150 h), both in the third year and related to optical and separation instrumental techniques, respectively.
The desired outcome was to enhance the students' general, specific and transversal skills (see Table 1) and practices in analytical chemistry in their final BS dissertations [34]. Test subjects were undergraduate students who intended to finish the Chemistry Bachelor by presenting their BS thesis before a chemistry teachers' panel. This experience was carried out at the student laboratories of the Analytical Chemistry Department at the Complutense University of Madrid (UCM). Table 1. General, specific, and transversal skills.

General
Recognize and analyze new problems and envisage strategies to solve them. Evaluate, interpret, and synthesize chemical data and information. Demonstrate a solid and balanced knowledge base on lab materials and practical skills. Safely handle chemical materials and recognize and assess the risks in the use of chemical substances and procedures of laboratory. Interpret data from observations and measurements in the laboratory in terms of their significance and the theories that support them. Develop appropriate scientific measurement and experimentation practices.

Specific
Apply to chemical analysis the knowledge acquired in the study of chemical equilibrium. Apply to chemical analysis the fundamentals of the main technique's analysis and separation instruments.
Recognize analytical chemistry as a metrological science that develops, optimizes, and applies measurement processes aimed at obtaining quality analytical chemist information. It is worth noting that the learning objectives of this type of project assume that students will not only gain theoretical knowledge, but also develop the practical skills and critical thinking abilities essential for success in the field of Analytical Chemistry, as pointed out in Table 1. These objectives were as follows:

1.
Gain a comprehensive understanding of various analytical techniques in monitoring water quality.

2.
Identify and analyze key chemical parameters that affect freshwater aquarium water quality.

3.
Learn the principles of the calibration and standardization of analytical instruments, ensuring accurate and reliable measurements.

4.
Develop skills in proper sample collection, preservation, and preparation techniques to maintain the integrity of the water samples during analysis.

5.
Understand the importance of quality assurance and control in analytical chemistry to minimize errors and ensure data accuracy. 6.
Practice interpreting analytical data, understanding the implications of different parameter levels on aquarium health and making informed decisions based on the results. 7.
Develop problem-solving abilities to troubleshoot and resolve analytical challenges that may arise during the course of the project. 8.
Demonstrate a commitment to laboratory safety, understanding potential hazards associated with specific analytical techniques and how to mitigate them. 9.
Enhance research skills by conducting literature reviews on aquarium water quality, staying up-to-date with current research, and effectively communicating findings via reports and presentations. 10. Collaborate effectively in a team environment, assigning roles and responsibilities, and working together to achieve project goals.
11. Explore the impact of aquariums on the environment and explore ways to maintain water quality in an eco-friendly manner. 12. Apply the analytical chemistry principles learned in the course to real-world applications and problem-solving in the field of aquarium keeping and water-quality management. 13. Understand the ethical considerations involved in aquarium keeping, including responsible pet ownership, conservation, and promoting animal welfare.
For the students' assessment, their tutor assessed the student's final dissertation, taking into account different criteria (see Table 2), from 0 to 10. In addition, the students had to procced with an oral defense of their dissertation in front of a panel of Chemistry teachers of different areas of Chemistry. The overall goal was the public presentation and defense of the study. During this act, the teachers explored, with the candidate, the methodology, findings and conclusions revealed by the student's study. In this way, the examiners could obtain a more extensive insight into the student's work; they assessed, independently, the student' oral defense and dissertation using the descriptors listed in Table 2, from 0 to 10 as well. The final mark was given by applying the following weighting: 30% from tutor and 70% regarding examiners (35% written dissertation and 35% oral presentation).

Theoretical Framework
Through the literature review, the students were expected to develop the ability to select the key water parameters for monitoring, and choose appropriate analytical methods for their analysis. However, it was important for the teacher(s) to provide valuable guidance regarding the physicochemical parameters, organic matter, the nitrogen cycle, and nutrient considerations.

Physicochemical Parameters
To begin with, temperature is an important parameter for aquarium fishes, plants, and bacteria. It influences both the dissolved oxygen (DO) and toxicity (ionization) of ammonia. Thus, high temperatures have less DO and more unionized (toxic) ammonia [35].
Regarding pH, the aquarium pH is controlled by the carbonate buffer system [24]. Many water quality parameters are affected, including ammonia and ammonium concentrations [36]. Therefore, pH levels should be monitored frequently.
Concerning the salinity-the concentration of salts in water-electrical conductivity and/or total dissolved solids are habitually used for the measurement of the total amount of nutrient salts in the water. It is influenced by temperature and pH, and it affects both freshwater fish and plants [35].
With respect to the solids in water, these can be determined as total solids, suspended solids, dissolved solids, fixed solids, volatile solids and/or settleable solids. A high TSS (total suspended solids) concentration is indicative of poor water quality. Therefore, this parameter can be considered as an indicator of the water turbidity and/or contamination [37].
Finally, one of the key water quality parameters is water hardness. Calcium and magnesium salts, which generally are present in water as bicarbonates or sulfates or chlorides, cause water hardness. Carbonate hardness, also known as alkalinity, is a measure of the buffering capacity of water. It has a unique relationship with pH, impacting the pH level. In fact, the carbonate and bicarbonate present in the water can neutralize the acids created naturally in the aquarium and keep the pH at the constant values required for fish, plants, and bacteria [35].

Organic Matter
The amount of organic matter in the aquarium can be measured by the estimation of the chemical oxygen demand. The biochemical oxygen demand (BOD) is the oxygen consumed by aerobic microorganisms as they break down organic material. While BOD shares similarities with COD in terms of measuring organic compounds, it primarily focuses on quantifying biologically active organic matter. Even if COD is less associated with natural processes and is completely artificial, COD tests offer rapid analysis and provide a result that could be used as a basis for calculating a reasonably accurate and reproducible estimate of the oxygen-demanding properties of water samples. The BOD test requires five days of analysis; it is difficult to reproduce and is also affected by the temperature, water pH, type of inorganic substances in water, and amount and type of organic substance.
In addition, the presence of certain kinds of microorganisms affects the growth of aerobic bacteria [38]. In this teaching-learning experience, the COD test, and the permanganate index (IMn) method in particular, was selected as an indicator of the potential of water to be polluted by organics and reduced inorganic substances, such as nitrite, ferrite and sulfide [39]. The COD test yields the total quantity of oxygen that is required for the oxidation of organic material to carbon dioxide. High COD values are not desirable for fish health and aquatic habitats. Therefore, it must be kept low by maintaining the effective filtering of solids and the regular cleaning of the water tank [40].

Nitrogen Cycle
Taking into consideration the nitrogen cycle, nitrogen is one of the most critical water parameters because it is required by all life and is part of all proteins. All four forms of nitrogen involved in the aquarium cycle (NH 3 , NH 4 + , NO 2 − , NO 3 − ) can be determined using colorimetric and spectrophotometric methods. Ammonia and nitrite are toxic to fish, while the nitrate content is the reason for algae growth and plant nutrients [35]. Figure 2 shows a schematic representation of the N cycle in an aquatic system. while the nitrate content is the reason for algae growth and plant nutrients [35]. Figure 2 shows a schematic representation of the N cycle in an aquatic system.

Nutrients
Regarding macro and micronutrients, plants within the aquatic system need several nutrients; these are required for the enzymes that facilitate the photosynthesis needed for both growth and reproduction. Thus, phosphorous and potassium are needed by the plants in relatively large amounts, while iron is a micronutrient required in trace amounts. Over time, a perfectly balanced aquarium may become deficient in certain nutrients, most often iron or potassium. Deficiencies in these nutrients are a result of the composition of the fish feed, which does not necessarily provide everything needed for plant growth [35].

Design and Setup of a Freshwater Aquarium
A freshwater aquarium with a capacity of about 55 L can be set up using a glass aquarium as a fish tank (55 × 30 × 34.8 cm) to facilitate the work of the students; see Figure  3. A filter system, a water pump with a flow regulator, a heater (50 W) with a thermostat, and a thermometer must be added. The mentioned filter could incorporate a set of filtrating cartridges, such as fiber (retaining the smaller polluting particles),activated carbon (removing dissolved organic waste, bad odors and toxins), coarse foam (promoting the biological decomposition of pollutants), and some natural product with a high porosity, like clay pebbles (providing optimum conditions for bacterial growth). In other words, the filtration system should carry out the biological, mechanical, and chemical filtration of the fresh water, both at the surface and on the bottom of the aquarium. Likewise, the tank should be constantly aerated and may include LED lights directly above the aquarium that may be continuously illuminated, e.g., 12 h per day.

Nutrients
Regarding macro and micronutrients, plants within the aquatic system need several nutrients; these are required for the enzymes that facilitate the photosynthesis needed for both growth and reproduction. Thus, phosphorous and potassium are needed by the plants in relatively large amounts, while iron is a micronutrient required in trace amounts. Over time, a perfectly balanced aquarium may become deficient in certain nutrients, most often iron or potassium. Deficiencies in these nutrients are a result of the composition of the fish feed, which does not necessarily provide everything needed for plant growth [35].

Design and Setup of a Freshwater Aquarium
A freshwater aquarium with a capacity of about 55 L can be set up using a glass aquarium as a fish tank (55 × 30 × 34.8 cm) to facilitate the work of the students; see Figure 3. A filter system, a water pump with a flow regulator, a heater (50 W) with a thermostat, and a thermometer must be added. The mentioned filter could incorporate a set of filtrating cartridges, such as fiber (retaining the smaller polluting particles),activated carbon (removing dissolved organic waste, bad odors and toxins), coarse foam (promoting the biological decomposition of pollutants), and some natural product with a high porosity, like clay pebbles (providing optimum conditions for bacterial growth). In other words, the filtration system should carry out the biological, mechanical, and chemical filtration of the fresh water, both at the surface and on the bottom of the aquarium. Likewise, the tank should be constantly aerated and may include LED lights directly above the aquarium that may be continuously illuminated, e.g., 12 h per day.
The type and number of fish depends on the tank capacity, because any overload of the system would affect the water parameters and then the health of the aquarium. For instance, three guppies (Poecilia reticulata) and two peppered catfish (Corydoras paleatus) can be introduced into the tank, and flake food can be added daily in measured amounts. Since they are highly adaptable and thrive in several different environmental and ecological conditions, guppies are one of the most popular freshwater aquarium fish species. In addition, guppies are livebearers, and they reproduce very easily. In fact, after a month, several juvenile guppies could be born. On the other hand, peppered catfish are highly appreciated by aquarists for their eating habits, as they continuously search for all kinds of edible organic matter, thus removing any matter that can decompose and contaminate the water; they thus indirectly perform a cleaning task. The aquarium also may contain small ornamental plants such as Anubias barteri, which are one of the more popular and resilient of the freshwater aquarium plants. The type and number of fish depends on the tank capacity, because any overload of the system would affect the water parameters and then the health of the aquarium. For instance, three guppies (Poecilia reticulata) and two peppered catfish (Corydoras paleatus) can be introduced into the tank, and flake food can be added daily in measured amounts. Since they are highly adaptable and thrive in several different environmental and ecological conditions, guppies are one of the most popular freshwater aquarium fish species. In addition, guppies are livebearers, and they reproduce very easily. In fact, after a month, several juvenile guppies could be born. On the other hand, peppered catfish are highly appreciated by aquarists for their eating habits, as they continuously search for all kinds of edible organic matter, thus removing any matter that can decompose and contaminate the water; they thus indirectly perform a cleaning task. The aquarium also may contain small ornamental plants such as Anubias barteri, which are one of the more popular and resilient of the freshwater aquarium plants.
Incorporating a freshwater aquarium as a didactic tool demands careful consideration, with strong commitment required from the educators and/or technicians involved for its successful implementation. Beyond the initial setup and routine maintenance, this commitment becomes especially crucial when navigating factors like the school's summer break or extended holidays. During these periods, special arrangements and dedicated attention are imperative in order to safeguard the well-being of the aquatic ecosystem and its inhabitants. Close collaboration between educators and technicians is essential in order to establish effective contingency plans, delegate responsibilities, or even explore the possibility of temporary foster care for the aquarium during extended breaks. By proactively addressing these challenges, the educational potential of the aquarium can be fully realized, thus fostering a captivating learning experience.

Identification of Water Quality Parameters
The students guided by their supervisor must review the scientific literature to identify the most important water quality parameters and their range , which to some extent are determined by the fish species and the types of plants. Indeed, each organism in the aquarium (fish, plant, and bacteria) possesses a specific tolerance concentration range [35]. Therefore, a concentration level compromise is required to maintain a balanced ecosystem. As an example, the parameters that may be proposed by the students for the water quality measurements are temperature, pH, electrical conductivity (EC), total suspended Incorporating a freshwater aquarium as a didactic tool demands careful consideration, with strong commitment required from the educators and/or technicians involved for its successful implementation. Beyond the initial setup and routine maintenance, this commitment becomes especially crucial when navigating factors like the school's summer break or extended holidays. During these periods, special arrangements and dedicated attention are imperative in order to safeguard the well-being of the aquatic ecosystem and its inhabitants. Close collaboration between educators and technicians is essential in order to establish effective contingency plans, delegate responsibilities, or even explore the possibility of temporary foster care for the aquarium during extended breaks. By proactively addressing these challenges, the educational potential of the aquarium can be fully realized, thus fostering a captivating learning experience.

Identification of Water Quality Parameters
The students guided by their supervisor must review the scientific literature to identify the most important water quality parameters and their range, which to some extent are determined by the fish species and the types of plants. Indeed, each organism in the aquarium (fish, plant, and bacteria) possesses a specific tolerance concentration range [35]. Therefore, a concentration level compromise is required to maintain a balanced ecosystem. As an example, the parameters that may be proposed by the students for the water quality measurements are temperature, pH, electrical conductivity (EC), total suspended solids (TSS), water hardness, chemical oxygen demand (COD), ammonium, nitrite, nitrate, phosphate, iron(III), and potassium contents [24,29,35,38,40].
It is convenient to understand the effect of each parameter on the organisms in the aquarium, as well as the possible interrelation between them. For instance, carbonates and bicarbonates influence the pH value, which seriously affects the conversion of non-toxic ammonium ion to toxic ammonia. Both ammonia and ammonium ion can be oxidized to nitrites, which are toxic to fish at low concentrations, and nitrites can be oxidized to nitrates, which are generally tolerated by living organisms [14]. In particular, shifts in the water's pH have an impact on the biological activity of the nitrifying bacteria and their Sustain. Chem. 2023, 4 290 ability to convert ammonia and nitrite [35]. Thus, the ammonia toxicity, expressed as the total ammonia (the sum of NH 3 and NH 4 + ), increases with the water's pH [36].

Water Analysis
The continuous monitoring of water parameters is essential to ensure the well-being of aquatic life [31][32][33]. Consequently, it becomes imperative to measure several key parameters using standard methods to maintain the required water quality. These methods must be selected by considering which instruments of measurement are available, their relative ease of use, the precision and sensitivity required, the number of samples and/or matrix effects. Temperature, pH, and EC can be measured using a thermometer, a portable digital pH meter, and a handheld conductivity meter, respectively. Total suspended solids (TSS) can be determined via the vacuum filtration of water aliquots (250 mL) through weighed standard glass-fiber filters (pore size 0.45 µm), the evaporation of the filtrates in an oven at 103-105 • C for one hour, and then filter-cooling in a desiccator until constant weight is reached [37].
The total water hardness, and the total calcium and magnesium content, can be measured via complexometric titration, using samples buffered at pH 10, with a standard EDTA solution (0.001 M) [41,42]; meanwhile, COD is determined via the permanganate index (IMn) method, performing a back-redox titration with sodium oxalate (0.005 M) and a previously standardized KMnO 4 solution (0.002 M) [39,43].
Spectrophotometric methods can be employed for the quantitative analysis of ammonium using the Nessler method, nitrate, nitrite, phosphate using the vanadomolybdophosphoric acid colorimetric method, and iron(III) using the thiocyanate method [37,44]. For absorbance measurements in the experimental colorimetric determinations, a UV-Vis spectrophotometer could be employed. Potassium can be determined via flame emission photometry at a wavelength of 766 nm [37] using an atomic absorption spectrometer operating in the flame emission mode. Comprehensive experimental analysis procedures for a freshwater aquarium can be found in Appendix A. This section provides detailed insights into the methods employed for the study, allowing readers to gain a deeper understanding of the research process. The information presented in Appendix A is essential for replicating the experiments and verifying the results obtained in the study.
Additionally, commercial aquarium water testing kits are available to measure the pH level, total hardness and carbonate hardness ( • dH), and ammonium/ammonia, nitrite, nitrate, phosphate, silicates (silicic acid), iron, copper, and oxygen concentrations (mg L −1 ). These aforesaid tests, see Figure 4, must be carried out according to the supplier instructions [45]. External calibration must be applied for quantifying purposes in the instrumental methods, and calibration curves should be prepared. Then, linearity should be evaluated

Data Analysis and Statistical Treatment
External calibration must be applied for quantifying purposes in the instrumental methods, and calibration curves should be prepared. Then, linearity should be evaluated in terms of the determination coefficients (R 2 ), and calibration curve equations should be obtained via the analysis of analytical signals via least squares linear regression.
Each measurement should be performed in quintuplicate. The results obtained from the aquarium water analysis should be expressed as the mean values and the 95% confidence intervals of the test results. The precision and repeatability can be estimated using relative standard deviation (RSD, %). The identification and rejection of outliers may be performed using the Dixon's Q-test at a 95% confidence level.

Experimental Implementation
The laboratory procedure can be envisioned following the instructional scaffolding depicted in Figure 5, guided by a comprehensive analysis of the literature, meticulous selection, optimization of the analytical methods, proficiency in scientific instrumentation handling, and the valuable formative feedback provided upon the calculation and evaluation of the acquired data. By sharing results and engaging in collaborative decision making, an engaging and instructive laboratory experience is fostered, further promoting student autonomy in the learning process. When the aquarium is ready, water aliquots should be taken, and the water quality parameters should be determined based on the analytical methods selected and optimized by the students. Table 3 summarizes a proposal of the analytical parameters that can be measured, and their corresponding methods. As can be observed, classical (gravimetry and titration) and instrumental (potentiometry, spectrophotometry UV-Vis and atomic emission) analytical methods can be performed.  In terms of setting up the freshwater aquarium, it is advisable to test the pH and hardness of the water after filling the tank with tap water, prior to introducing any fish. the preferences of the fish must be kept in mind; for example, guppies prefer a hard-water aquarium (10-20 • dH) with a temperature between 22 and 28 • C and pH levels in the range of 6.5-8.0 [46]. Another issue to be considered is the water source, which has an impact on the water chemistry, because tap or municipal water is often treated with different chemicals, e.g., chlorine and chloramines, to remove pathogens. These chemicals are toxic to fish, plants, and bacteria, and they can seriously affect the aquatic ecosystem [35]. Hence, in the case of tap water, the students must treat it before it is used by adding a commercial dechlorinating chemical and a water conditioner to increase the carbonate hardness. In addition, commercial concentrated aquarium bacteria should be added to diminish the presence of harmful ammonia and nitrite, and to maintain water quality, making it possible to introduce fish after setting up the new aquarium. In addition, the temperature and pH must be frequently monitored by the students to ensure the optimum fish living conditions. When the aquarium is ready, water aliquots should be taken, and the water quality parameters should be determined based on the analytical methods selected and optimized by the students. Table 3 summarizes a proposal of the analytical parameters that can be measured, and their corresponding methods. As can be observed, classical (gravimetry and titration) and instrumental (potentiometry, spectrophotometry UV-Vis and atomic emission) analytical methods can be performed. Afterwards, the obtained results need to be analyzed, evaluated, and discussed by the students in accordance with the recommended optimum levels for a freshwater aquarium. If necessary, they should propose corrective actions to address any deviating values.

Experimental Activities
This section presents comprehensive details and experimental findings from the dedicated capstone project conducted during the 2020/2021 academic year, as the 2019/2020 project was severely impacted by the COVID-19 pandemic, leading to the inadequate development of activities. Despite the challenges posed by the pandemic, the 2020/2021 project was successfully executed, and this section highlights the essential research conducted, shedding light on effective aquarium water quality monitoring techniques.
Firstly, the first step was the implementation of the freshwater aquarium with the dimensions outlined in Section 2.3.1. The tank was filled with tap water (40 • 27 02 N 3 • 43 04 O/40.45052778, −3.7186111), which was tested for pH and hardness before introducing fish into the tank. After the measurements were taken by the students, the pH was determined to be 7.53 ± 0.04, and the total hardness was (19 ± 2) mg CaCO 3 L −1 (≈ 1.1 • dH). The tap water was treated before being used by adding a commercial dechlorinating chemical, and a water conditioner was used to increase the carbonate hardness. In addition, commercial concentrated aquarium bacteria were added to diminish harmful ammonia and nitrite, and to maintain the water quality, making it possible to introduce fish immediately after setting up the new aquarium. In addition, the temperature and pH were frequently monitored by the students to ensure the optimum fish living conditions (temperature of 25 • C and pH of 8.0 maximum). Once the aquarium was set up, water aliquots were taken after a period of 15 days, and they were stored at −20 • C until analysis.
The water quality parameters, see Table 4, were determined based on the analytical methods selected and optimized by the students. In addition, several calibration curves (n ≥ 5) were prepared in the following concentration ranges: 0.  For comparison purposes, water analysis was also carried out by using a commercial test kit, i.e., the JBL ProAquaTest Lab ® kit from JBL GmbH & Co. KG (Neuhofen, Germany) [45], which was employed for measuring the following parameters: pH level in the range of 3.0-10.0 and in the range of 7.4-9.0, the total hardness and carbonate hardness ( • dH), and the ammonium/ammonia, nitrite, nitrate, phosphate, silicates (silicic acid), iron, copper, and oxygen concentrations (mg L −1 ). These assays, see Table 4, indicated acceptable oxygen and copper concentration levels, but a high silicate content. In general, the obtained results from both analytical methods and the test kit agreed. However, lower ammonium and iron contents were determined when using the kit assays.
The obtained results (Table 4) were thoroughly analyzed, evaluated, and discussed by the students, taking into consideration the recommended optimum levels for the freshwater aquarium [35,45,[47][48][49]. Please refer to Table 5 for further details. As can be observed in Table 4, data analysis from the analytical methods revealed a low level of water hardness and low TSS values, suggesting low water turbidity, an appropriate pH and salinity and adequate COD, indicating low organic matter content and/or water contamination, proper concentrations of both phosphate and iron, and low potassium levels. Although low nitrate and nitrite contents were determined, high ammonium concentrations were measured.
Although commercial tests provide quick and convenient water values, the precision of the results obtained using analytical methods led the student to propose the following actions: • Perform a 50% water change in the aquarium, while ensuring that the pH value of the tap water being added is checked beforehand. The pH of the tap water should not exceed the pH of the aquarium water. This corrective action will also help decrease the high silicate concentration identified using the test kit. It is important to note that ammonia and ammonium (NH 4 + ) exist in a chemical equilibrium in the aquatic system, with their relative concentrations determined by pH. Ammonia (NH 3 ) is the initial product of nitrogenous organic waste decomposition, and its presence often indicates the existence of such wastes [50]. Therefore, the ammonium ion content should be kept as low as possible to prevent conversion into NH 3 , which is highly toxic to fish. • Increase the carbonate and total hardness reconditioning of the freshwater by adding hydrogen carbonate conditioners and/or mineral salt mixtures. Carbonate hardness directly affects the pH level. However, it is worth mentioning that the commercial conditioner used resulted in an increase in the pH up to a maximum value of 8.5. Additionally, after water conditioning, an increase in electrical conductivity may also be expected.

•
Address the low potassium concentration levels by adding potassium fertilizers to the aquatic system. This action is necessary due to the observed burned spots on older leaves and poor plant vigor.

Attitude and Perception of the Students
Remarkably, via the students' written and oral dissertations, it was found that, first, in terms of content and organization, the students highlighted that the literature consulted was relevant and comprehensive and helped them to prepare their experiments based on published results, and the laboratory resources were well-organized and managed. Secondly, another positive issue pointed out by students in their final reports was their interaction with the teacher, who helped them master the chemistry skills they need to succeed, e.g., providing support in the form of outlines, recommended documents, templates, guides, storyboards, or key questions. Among other skills, data analysis and the evaluation of the water quality parameters, including the determination of potential adjustments to keep the aquarium in steady-state conditions, enhanced the development of the students' critical thinking. Certainly, given the impact of the regulation of the water parameters on the health of the aquatic life, the students improved their ability to select the appropriate techniques and methods in order to obtain accurate and reliable analytical results. Furthermore, regarding the impact of the experimental activities on the students' environmental awareness, these enabled the students to realize the contribution of analytical chemistry to preserving our natural resources and to achieving sustainable development, which can be supported with some other examples collected in their literature, such as alternative food production systems based on aquaculture and aquaponics [35].
A similar experience was conducted for 29 pre-service teachers at the Faculty of Education (UCM), although only using commercial water testing kits; after completing a course devoted to the Essentials and Didactics of Chemistry, and participating in a satisfaction survey specifically designed for this purpose, the participants unanimously agreed with the assertion that, despite the required effort to implement an aquaponic system, this resource could greatly assist Elementary Education teachers in motivating their students to engage with science. In fact, the students expressed that their interest in learning new sustainable approaches and strategies had significantly increased. Among the comments regarding what intrigued them the most about working on the aquaponic cultivation project, the idea that they actively learned about alternative "sustainable methods" stood out, alongside the collaborative nature of the activity [51].

Conclusions
Via the aquarium water quality experience, students are presented with a unique opportunity to delve deeply into the chemical processes that underpin the maintenance of a healthy aquatic environment and their interconnectedness with broader ecological systems. This hands-on experience in a Final Degree Project context yields multiple benefits, allowing students to contextualize and apply fundamental scientific facts, concepts, and theories, while exploring classical and instrumental analytical techniques, such as gravimetry, titration, UV-Vis spectrophotometry, and emission photometry. These analytical techniques become relevant in the analysis of the physicochemical parameters crucial for water monitoring, including hardness, chemical oxygen demand, ammonium, nitrate, nitrite, phosphate, iron, and potassium concentrations. Moreover, beyond theoretical learning, actively engaging in the setup and maintenance of a freshwater aquarium empowers students to develop practical skills that find real-life applications. This interactive approach not only immerses students in the learning process, but also nurtures their ability to tackle real-world scenarios with confidence.
In essence, this paper offers adaptable content with which to promote sustainability across environmental, economic, societal, and educational dimensions by integrating aquaponics and/or freshwater aquariums into chemistry classes. These experiential learning opportunities have the potential to generate superior educational outcomes compared to traditional laboratory exercises, all while nurturing interdisciplinary learning. In the realm of Analytical Chemistry, students can acquire essential knowledge of vital analytical techniques and gain valuable insights into the unique challenges related to chemical analysis for aquatic ecosystems.

Data Availability Statement:
The data presented in this study are available upon request from the corresponding author.

Acknowledgments:
We acknowledge with gratitude the financial support of the Complutense University of Madrid (Spain).

Conflicts of Interest:
The authors declare no conflict of interest.

Appendix A
This appendix contains details of the analysis methods used for freshwater aquariums. • Principle: Ethylenediaminetetraacetic acid and its sodium salts (EDTA) form a chelated soluble complex when added to a solution of specific cations. When a small amount of Eriochrome Black T dye is added to an aqueous solution containing calcium and magnesium ions at a pH of 10.0, the solution becomes wine-red. If EDTA is added as a titrant, the calcium and magnesium will be complexed. When all of the magnesium and calcium has been complexed, the solution turns to a blue color, marking the end point of the titration. Nessler's reagent: dissolve 3.5 g of potassium iodide (KI) and 5 g of mercury(II) iodide in 25 mL of NaOH solution 5.0 M and dilute to 50 mL with milli-Q water. Stock ammonium chloride solution 1000 mg L −1 : dissolve 0.2965 g of anhydrous NH 4 Cl, dried at 100 • C, in water, and dilute to 100 mL. Standard ammonium chloride solution 10 mg L −1 : take a volume of 0.5 mL from the stock NH 4 Cl solution and dilute to 50 mL with milli-Q water. Wait 10 min and transfer to a 1 cm spectrophotometer cell and measure the absorbance at 400 nm. • Calculation: Construct a standard curve by plotting absorbance due to NH 4 + against the NH 4 + concentration of the standard. The ammonium concentration in the water sample is directly determined via linear interpolation using a calibration curve.

•
Principle: The determination of nitrate is performed using an ultraviolet (UV) technique that measures the absorbance of NO 3 at 220 nm. Since dissolved organic matter also may absorb at 220 nm and NO 3 -does not absorb at 275 nm, a second measurement made at 275 nm is required to correct the NO 3 absorbance value. Acidification with 1N HCl aims to prevent interference from hydroxide or carbonate. Chloride has no effect on the determination. • Calculation: For samples and standards, subtract two times the absorbance reading at 275 nm from the reading at 220 nm to obtain absorbance due to NO 3 -. Construct a standard curve by plotting absorbance due to NO 3 against the NO 3 concentration of the standard. Using the corrected sample absorbances to obtain sample concentrations directly from standard curve. metric determination, since this increases the intensity and stability of the color. A strong acid, such as nitric acid, must be present to suppress the hydrolysis of the iron(III) cation. Aspirate the calibration standards and water sample several times to secure a reliable average reading for each. Construct a calibration curve from the potassium standards. The water sample is analyzed via a direct-intensity measurement. • Calculation: The potassium concentration is determined via direct reference to the calibration curve. If necessary, dilute the sample and consider the dilution ratio in calculation.