Feeding Time Optimization Enhances Aquaponic Performance: Growth, Water Quality, and Nutrient Removal in Systems Integrating Cyprinus carpio and Lactuca sativa

Abstract

Feeding time is a critical but understudied factor influencing nutrient dynamics and overall productivity in aquaponic systems. This study examined the effects of two feeding schedules on growth performance of common carp (Cyprinus carpio L.), hydrochemical parameters, and the growth of lettuce (Lactuca sativa) cultivated in an integrated aquaponic system. Two 60-day trials were conducted over consecutive years under identical greenhouse conditions. Carp were fed either in the morning and early afternoon (T1: 08:00, 11:00, 14:00) or later in the day (T2: 11:00, 14:00, 17:00). Hydrochemical indicators, including dissolved oxygen, turbidity, ammonium ions (NH₄⁺), and nitrates (NO₃⁻), were continuously monitored through online measurement. Carp reared under T2 displayed significantly higher specific growth rate, final body mass, and improved feed conversion ratio (p < 0.05). The T2 variant also showed higher dissolved oxygen levels and lower turbidity compared to T1, indicating enhanced system stability. Although NH₄⁺ concentrations were higher and NO₃⁻ levels lower in T2, these differences did not compromise water quality due to efficient plant nutrient uptake. Lettuce grown under T2 exhibited greater stem and root development and higher biomass accumulation, suggesting improved nitrogen utilization linked to the NH₄⁺/NO₃⁻ ratio and enhanced root oxygenation. Overall, aligning feeding time with fish circadian rhythms improved fish performance, plant growth, and nutrient cycling efficiency. These findings demonstrate that feeding schedule is a key management factor capable of enhancing sustainability and productivity in aquaponic systems.

1. Introduction

In the coming decades, agriculture will face three major challenges: increasing food production by 70%, addressing hunger and poverty, and adapting to the impacts of climate change while using natural resources sustainably. Worldwide, more than one billion people suffer from malnutrition, making food security a global problem [1]. Soilless farming systems are already showing promising results globally. They ensure efficiency in food production, which is an essential step toward environmentally sustainable agriculture [2] and can be regarded as a tool for addressing the issue of food security [3]. Therefore, recirculating production systems such as aquaponics have attracted increasing attention as integrated solutions to these global challenges.

Among soilless farming approaches, aquaponic systems demonstrate extremely high water-use efficiency, significantly surpassing conventional agricultural practices [4]. This makes aquaponic technologies suitable for regions where water resources are limited and particularly important in the context of rising global food demand. This technology offers a promising solution to the problem of soil, air, and water pollution, contributing to food security [5]. Thus, aquaponics links water conservation, pollution reduction, and food security within a single production concept.

In aquaponic systems, plant cultivation and fish farming take place within a closed ecosystem, which prevents soil and groundwater contamination caused by the use of pesticides and fertilizers in traditional agriculture [6]. Thanks to this closed-loop design, aquaponics minimizes the risk of harmful chemicals entering the environment through wastewater. The system is nearly zero-waste, as even the small amount of waste produced can be composted or converted into useful products [7]. This is especially important for the circular economy, where every resource can be reused to reduce the environmental footprint. The efficient reuse of water and waste in aquaponics, however, depends on balanced plant nutrition and nutrient cycling within the system. Plants require 16 essential elements for growth and development, with carbon (C), hydrogen (H), and oxygen (O) obtained from air and water, while the rest are absorbed from the substrate through the roots [8]. Among these, nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and sulfur (S) are macronutrients needed in large quantities. To optimize the uptake and utilization of these elements, plants have developed complex absorption and regulatory mechanisms [9].

In greenhouse-based aquaponic production, light is a key factor influencing nutrient uptake through changes in its intensity, quality, and photoperiod. These light characteristics are largely determined by the season and time of day. The connection between light signals and nutrient balance in plants has been examined in numerous studies [10]. Plants use different photoreceptors to perceive light and activate various physiological responses [11]. Phytochrome receptors respond mainly to red and far-red light and play a role in many light-mediated processes [12], while cryptochromes perceive blue and ultraviolet light and are associated with circadian rhythm regulation and photomorphogenesis [13]. Light significantly affects nitrogen uptake, its transport, and its incorporation into organic compounds [14]. Photosynthesis directly controls nutrient absorption, and variations in light intensity lead to changes in sugar accumulation, which in turn regulates the uptake of nitrates and ammonium ions [15]. In addition, day length influences physiological responses such as germination, organ differentiation, and flowering [16]. Thus, daily fluctuations in light and photoperiod create temporal patterns in nutrient demand.

For vegetable crops, light plays a crucial role in growth and development. Regulating nutrient supply in accordance with the available light environment can increase yields and the economic value of plants, as well as improve their taste and quality [17]. Therefore, adjusting nutrient delivery to match the light conditions during specific times of day offers an effective way to achieve high-quality and sustainable production in aquaponic systems. In aquaponic setups, one practical way to influence the timing of nutrient availability is through the feeding schedule of fish, which largely determines when dissolved nitrogen compounds are released into the water.

Common carp (Cyprinus carpio L.) is a robust, omnivorous species with a long history in freshwater aquaculture and has recently been integrated into carp–lettuce aquaponic systems that show high nutrient-use efficiency and favorable environmental performance [18,19]. Lettuce (Lactuca sativa L.) is among the most widely used crops in aquaponics because of its short growth cycle, high nitrogen uptake capacity, and consistent market demand, making the carp–lettuce combination a well-established model for studying nutrient dynamics and production efficiency in aquaponic systems [18,19]. Growth performance and circadian rhythms of behavior, digestion and metabolism in fish are strongly affected by feeding time, which can act as a powerful zeitgeber in aquaculture systems [20,21]. Experimental studies show that aligning feeding with the species’ active phase can improve specific growth rate and feed conversion efficiency by synchronizing digestive physiology and food anticipatory activity with feed availability [20,21]. Water quality and nutrient removal in aquaponic systems are mainly affected by the balance between fish waste production, microbial transformations and plant uptake, as well as by hydroponic design and water management [22,23]. Effective removal of dissolved nitrogen and phosphorus is essential to prevent toxic accumulation and to maintain a stable environment for both fish and plants [22,23].

A review of existing literature revealed that research in this area is limited. One study examined the effects of feeding frequency and photoperiod on water quality and productivity in this type of environmentally friendly system [24], but it did not isolate or compare different feeding times during the day. To our knowledge, no specific studies have yet quantified how feeding time affects hydrochemical dynamics and the joint growth of fish and plants in integrated aquaponic systems, highlighting a clear knowledge gap and the need for research in this direction.

Therefore, the aim of the present study was to compare the influence of feeding time in carp (C. carpio L.) reared in an aquaponic system on selected hydrochemical parameters and the growth characteristics of fish and plants cultivated together.

2. Materials and Methods

2.1. Experimental Aquaponic System

The planned experiments were carried out at the Training and Experimental Aquaculture Base of the Fisheries and Aquaculture Section, Agricultural Faculty, Trakia University. The aquaponic recirculating system consisted of 10 fish-culturing tanks (Figure 1). The filtration unit of the system included a mechanical filter (settler), a moving-bed bioreactor, and 8 plant-cultivation tanks. The pump ensuring water recirculation was located in the sump tank. The effective volume of the fish-rearing tanks was 0.3 m³. The water flow rate in the fish section during the trial was 15 L·min⁻¹. Every day, the bottoms of the fish tanks and the filters were cleaned by opening the drain valve located at their base. Water losses amounted to 10% of the total system volume. This amount of water was replenished daily with fresh water. A stable, uniform water temperature in both the fish tank and the raft tank was maintained using two 2000 W submersible heaters installed in the sump and controlled by a thermostat with a temperature probe. The water temperature was maintained at 25 °C throughout the trial. Air temperature was kept constant at 20 °C during both trials using an air conditioner (Fujitsu AOYG30LFT, Kawasaki, Japan).

2.2. Experimental Fish

Stock material in good health condition was transported from the Fisheries and Aquaculture Institute in Plovdiv to the aquaculture research facility (Agricultural Faculty, Trakia University, Stara Zagora, Bulgaria). The initial weight of the common carp (Cyprinus carpio L.) at the start of cultivation was 0.270 ± 0.02 kg (min weight 0.220 kg, max weight 0.320 kg) in the first trial and 0.271 ± 0.03 kg (min weight 0.217 kg, max weight 0.330 kg) in the second. For feeding the carp in both trials, a granulated feed, Aqua Garant (Garant Tiernahrung Gesellschaft m.b.H., Austria), was used, administered three times per day with a pellet size of 4 mm.

Table 1. Composition of the feed used in the experiments.

Nutritional Component	Unit of Measure	Content
Crude protein	%	34
Crude fiber	%	3.5
Crude fat	%	12
Phosphorus	%	1.25
Vitamin A	IU·kg−1 × 1000	10
Vitamin D3	IU·kg−1	1500
Vitamin E	mg	175
Digestible energy	MJ	15

shows the composition of the feed used in the experiments. The stocking density during both experiments was 33 pcs.m⁻³.

To determine fish weight, the carp were weighed weekly during the experiment using a technical scale with an accuracy of ±0.1 g. The specific growth rate and the feed conversion ratio were calculated using the following formulas [25]:

$$\mathrm{S}\mathrm{G}\mathrm{R}={\displaystyle \frac{(\mathrm{L}\mathrm{n}\mathrm{W}\mathrm{f}-\mathrm{L}\mathrm{n}\mathrm{W}\mathrm{i})}{\mathrm{n}}}\times 100$$

where SGR—specific growth rate, %; Wi—initial weight, g; Wf—final weight, g; n—number of days.

$$\mathrm{F}\mathrm{C}\mathrm{R}={\displaystyle \frac{\mathrm{f}\mathrm{e}\mathrm{e}\mathrm{d}\mathrm{g}\mathrm{i}\mathrm{v}\mathrm{e}\mathrm{n}}{\mathrm{w}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}\mathrm{g}\mathrm{a}\mathrm{i}\mathrm{n}}}$$

where FCR—feed conversion ratio; feed given (g); weight gain (g).

2.3. Experimental Plants

Before the experiment was set up, lettuce seedlings (Lactuca sativa L.) were purchased from a nursery in Plovdiv. The plants were then transported to the Training and Experimental Aquaculture Facility, where they were placed into hydroponic baskets filled with expanded clay. At the beginning of both trials, the plants were at the same developmental stage—the third leaf stage. To ensure an appropriate micronutrient composition, the plants were supplemented with a specialized micro- and macronutrient solution suitable for this type of cultivation—Oligo Spectrum (containing DTPA iron, EDTA manganese, EDTA zinc, sodium molybdate, citric acid, potassium carbonate, and cobalt sulfate). To prevent possible nutrient deficiencies in the plants grown in the aquaponic system, the solution was applied according to the manufacturer’s recommendations.

2.4. Hydrochemical Parameters

During the experimental period, in order to monitor the relationship between fish feeding time and plant uptake of waste products, measurements of dissolved oxygen, ammonium ions, nitrates, and water turbidity were taken every 10 min. This was carried out using an online monitoring system equipped with appropriate probes and an 8-channel SC 1000 controller (Hach, Loveland, USA), where data from the electrodes were recorded and stored. Each week, the data from the controller were downloaded and saved to a portable storage device to ensure proper archiving.

2.5. Experimental Design

Two trials were conducted, each lasting 60 days. To ensure identical light conditions in the greenhouse, both experiments were carried out during the same time period of the year across two consecutive years:

• Trial 1: 1 October 2023–29 November 2023
• Trial 2: 1 October 2024–29 November 2024

Each trial was performed with 5 replicates for the fish and 8 replicates for the plants. For the purposes of the experiment, the same number of fish (100 individuals) and plants (80 individuals) were used, with no statistically significant differences in weight between individuals in the two experimental variants across both years.

In the two conducted trials, fish were fed according to the following schedules:

• T1—Trial 1 feeding schedule: 08:00, 11:00, 14:00
• T2—Trial 2 feeding schedule: 11:00, 14:00, 17:00

In both trials, the same feed was used, the composition of which is shown in Table 1. Process-based measurement of the parameters was carried out in the system’s expansion tank, where the connection between the fish section and the plant section takes place (Figure 1).

To validate the results obtained from the probes and to calibrate them, water samples were taken once per week and analyzed for the following parameters (

Table 2. Hydrochemical parameters analyzed using laboratory methods for the purpose of calibrating and validating the obtained results.

Parameter	Method	Method Number	Range
Turbidity	Infrared light measurement at 860 nm, according to 7 and ISO 7027 [26]	747	0–400 NTU
Dissolved oxygen	Optical measurement with LDO	–	–
Ammonium ions	Nessler method	8038	0.02–2.50 mg·L−1
Nitrates	Cadmium reduction method	8039	0.3–30 mg·L−1

).

At the end of the experiment, the following indicators were determined for the fish:

• Specific growth rate (SGR) (%.day⁻¹)
• Final live weight (kg)
• Feed conversion ratio (FCR)

Fish weight was measured using a technical scale with an accuracy of ±0.1 g. These parameters were calculated using the formulas presented above.

At the end of the experiment, the following indicators were determined for the plants:

• Stem length (cm)
• Root length (cm)
• Plant mass (kg)

The first two parameters were measured linearly, and the latter with a technical scale with an accuracy of ±0.1 g.

2.6. Data Processing

The results were statistically analyzed using one-way ANOVA (MS Office, 2010). When the factor showed a significant effect, differences between the mean values in the groups were evaluated using Fisher’s Least Significant Difference (LSD) test at a confidence level of p < 0.05.

3. Results

3.1. Growth Parameters in Common Carp (C. carpio L.)

The percentage difference in the specific growth rate (SGR) between the two groups was 6.78% in favor of the carp in T2. The difference in SGR between the carp from the two experimental variants was statistically significant (p < 0.05). The difference in final live weight at the end of the experiment was 2.32% (

Table 3. Growth parameters of the cultivated carp.

	T1	T2
Specific growth rate of carp (C. carpio L.) during the experiments, SGR %.day−1	0.59 ± 0.01	0.63 ± 0.01 *
Final live weight of carp (C. carpio L.) at the end of the experiment, kg;	0.388 ± 0.004	0.397 ± 0.004 *
Feed conversion ratio of carp (C. carpio L.) at the end of the experiment.	1.6 ± 0.02	1.5 ± 0.02 *

Asterisk (*) denotes a significant difference at p < 0.05.

) in favor of the fish in treatment T2, and this difference was also statistically significant (p < 0.05). These results support the hypothesis that feeding time is a critical factor for optimizing fish growth in aquaponic systems. Optimal fish growth is a key indicator of the efficiency of any aquaculture system.

The feed conversion ratio (FCR) is a key indicator of feeding efficiency in aquaculture. A lower FCR is desirable, as it indicates more efficient utilization of feed resources. The percentage difference in FCR between the groups was 6.25% in favor of the fish from T2 (Table 3). This means that the fish in the T2 group had better efficiency in converting the supplied feed into body mass.

3.2. Hydrochemical Parameters in Aquaponics

Water turbidity in an aquaponic system is a key indicator of the quality of the environment in which fish and plants coexist. It results from the presence of suspended particles such as feed residues, metabolic waste, and microorganisms. The lower turbidity observed in the tanks from treatment T2 (5.78 compared to 5.9 in T1) (

Table 4. Hydrochemical parameters during trial.

	T1	T2
Mean dissolved oxygen concentration in the water during the experiments, mg·L−1	8.55 ± 0.02	8.67 ± 0.02 *
Mean water turbidity during the experiments, NTU	5.9 ± 0.01	5.78 ± 0.01 *
Mean ammonium ion concentration in the water during the experiments, mg·L−1	0.11 ± 0.006	0.22 ± 0.008 *
Mean nitrate concentration in the water during the experiments, mg·L−1	1.4 ± 0.12	0.7 ± 0.14 *

Asterisk (*) denotes a significant difference at p < 0.05.

) indicates better processing of organic matter within the system and improved integration between fish and plants. The difference for this parameter was 2.03% in favor of T2 tanks, and it was statistically significant (p < 0.05).

The concentration of ammonium ions is often a direct result of fish metabolism, as NH₄⁺ is excreted through the gills and urine. In treatment T2, the later feeding schedule led to higher metabolic activity and increased release of NH₄⁺ into the system, as evidenced by the data showing a 100% difference between the two variants, a statistically significant difference (p < 0.05) (Table 4).

The percentage difference in nitrate (NO₃⁻) concentration between the groups was 50%, with T2 values (0.7 mg/L) being half those found in the tanks of treatment T1 (1.4 mg/L) (Table 4). This substantial difference indicates changes in nitrogen processing within the aquaponic system and highlights the influence of different feeding schedules.

The dissolved oxygen concentration in the T2 tanks was 1.40% higher compared to T1 (p < 0.05), indicating better oxygen saturation of the water under this feeding schedule (Table 4).

3.3. Growth Parameters in Lettuce (Lactuca sativa L.)

For treatment T1, the increase in stem length was 9.6 cm (from 11.9 cm at the beginning to 21.5 cm at the end), while for plants in treatment T2 it was 10.72 cm (from 11.08 cm to 21.8 cm). The percentage increase in stem length was considerably higher in T2 plants (96.75%) compared to T1 (80.67%), although the difference in final stem length was not statistically significant (p ≥ 0.05) (

Table 5. Growth parameters of the plants.

	T1		T2
	Start of Trial	End of Trial	Start of Trial	End of Trial
Mean stem length of the plants at the beginning and end of the experiments, cm	11.09 ± 0.9	11.08 ± 1.1	21.5 ± 0.8	21.08 ± 1.2
Mean root length of the plants in the two experimental variants, cm	9.4 ± 0.32	9.2 ± 0.35	25.6 ± 1.3	27.1 ± 2.5
Mean plant mass in the two experimental variants, kg	0.052 ± 0.001	0.051 ± 0.002	0.096 ± 0.001	0.101 ± 0.003 *

Asterisk (*) denotes a significant difference at p < 0.05.

).

The initial root length was 9.4 cm for T1 and 9.2 cm for T2, with no statistically significant difference (p ≥ 0.05). The absolute root growth at the end of the trial was 16.2 cm for T1 (25.6–9.4 cm) and 17.9 cm for T2 (27.1–9.2 cm) (Table 5). The percentage root growth was 172.34% for T1 and 194.57% for T2.

The percentage increase in plant weight was 84.62% for T1 and 98.04% for T2. The final difference in plant weight between T1 and T2 was 4.95% in favor of T2, and this difference was statistically significant (p < 0.05) (Table 5).

4. Discussion

One of the key aspects of circadian rhythms is their interaction with environmental conditions such as light, temperature, and food availability. The daily expression patterns of clock genes in trout liver are influenced by these environmental cues, with light mainly affecting clock1a and per1, and food modulating rev-erbβ-like. Studies on juvenile rainbow trout (Oncorhynchus mykiss) have shown that the liver circadian clock is not only entrained by the light–dark cycle but also by feeding time and temperature cues. This demonstrates that the circadian oscillator in trout liver responds to multiple synchronizing factors, highlighting the strong interplay between circadian rhythms and external environmental conditions [27].

For example, in common carp, it is well known that the highest metabolic activity often coincides with times of the day when water temperature and light intensity are elevated—a typical phenomenon in temperate climates [28]. This aligns with the findings of [29], who emphasized that lighting conditions strongly affect the physiological status of common carp, demonstrating that light is a key environmental driver shaping their metabolic responses.

This increased activity leads to more efficient digestion and improved assimilation of nutrients. In the context of aquaponic systems, properly aligning feeding time with the natural circadian rhythms of fish can greatly enhance not only fish growth but also the overall efficiency of the system [30]. The received results support the hypothesis that feeding time is a critical factor for optimizing fish growth in aquaponic systems. Higher fish growth is a key indicator of the efficiency of any aquaculture system. Optimized feeding schedules not only improve fish performance but also support water purification through the plant component. This conclusion is supported by [31], who showed that the strong influence of lighting and tray position on plant growth underscores the need for careful system design optimization to achieve maximum productivity and economic efficiency.

The balance between the release of waste products by fish and the ability of plants to absorb them is critical for maintaining a healthy environment in an aquaponic system [32]. Although the growth rates between T1 and T2 were similar, the better performance of carp in T2 (fed later in the day) can be explained by the behavior of carp under autumn conditions. Fish may exhibit better appetite and metabolism in the afternoon, when light intensity is highest during the day. Applying more precise control over feeding time not only improves fish growth but also enhances resource efficiency by reducing feed losses.

Improving water quality by reducing uneaten feed not only limits the buildup of organic matter but also supports the holistic balance required in aquaponic systems, where fish, plants, and bacteria function as an integrated unit [33]. Since water quality is the primary factor affecting both fish welfare and plant productivity, minimizing waste directly contributes to healthier fish and more efficient nutrient uptake by plants. This alignment between optimized feeding practices and system ecology ultimately enhances overall system sustainability and performance [34].

The later feeding schedule in T2 (11:00, 14:00, 17:00) is more synchronized with the circadian rhythms of the fish, leading to more efficient metabolism and lower release of metabolic waste products. Another possible reason for the lower turbidity in T2 is that the plants in the system may have been able to absorb organic matter and nutrients more efficiently, contributing to a reduction in suspended solids in the water. The improved light availability and enhanced oxygenation observed under T2 conditions align with findings that dissolved oxygen levels and LED light exposure significantly influence plant growth parameters. Biochemical and physiological traits such as transpiration, stomatal conductance, chlorophyll and nitrate content, sugar levels, and rates of photosynthesis and respiration all vary markedly with changes in oxygen concentration, underscoring the importance of maintaining optimal water clarity and oxygen levels [35].

Additionally, the reduced turbidity in T2 improves the environment for the fish by lowering the risk of infectious diseases associated with high concentrations of organic particles or pathogens. Finally, better water transparency reduces the need for additional mechanical or biological filtration, making the system more energy-efficient and more sustainable overall.

The biological filtration system in an aquaponic setup plays a key role in the nitrification process, in which NH₄⁺ is sequentially converted into nitrites (NO₂⁻) and nitrates (NO₃⁻) through the activity of the nitrifying bacteria Nitrosomonas and Nitrobacter [36]. Higher accumulation of NH₄⁺ may result from several factors, including insufficient filter capacity, reduced microbial metabolic activity, or temporary fluctuations in water quality parameters.

The toxicity of ammonium ions depends not only on their concentration but also on other environmental factors such as water pH and temperature. Higher pH levels can promote the conversion of NH₄⁺ into NH₃ (ammonia), which is highly toxic to aquatic organisms. For example, at pH 7.0, most ammonia is present as NH₄⁺, whereas at pH 8.5 the balance shifts toward toxic NH₃, leading to respiratory stress, gill damage, and even mortality in cultured fish [37].

On the other hand, ammonium ions serve as an important nitrogen source for plants in aquaponic systems, and in moderate amounts, they can support plant growth. However, excessive levels of NH₄⁺ can have negative effects, triggering physiological stress, growth inhibition, and symptoms such as chlorosis due to nitrogen imbalance [38].

Nitrates (NO₃⁻) are the final product of the nitrification process, in which ammonium ions (NH₄⁺) are first oxidized to nitrites (NO₂⁻) by Nitrosomonas, and then nitrites are further converted into nitrates by Nitrobacter [39]. The lower NO₃⁻ concentration in treatment T2 suggests that this feeding schedule influenced either the rate of nitrification or the ability of plants to assimilate available nitrates.

The higher oxygen levels in T2 contributed to more efficient metabolism and improved digestion in carp, which explains the better specific growth rate (SGR) and final body weight observed in this variant. Adequate dissolved oxygen reduces stress and enhances immune function in fish, whereas low oxygen levels can lead to hypoxia, slow growth, and reduced feed efficiency [40]. Thus, the elevated oxygen concentration in T2 likely allowed the fish to maintain higher activity and better feed utilization, resulting in greater final biomass.

Additionally, oxygen is essential for root respiration and nutrient uptake in plants. The higher O₂ levels in T2 likely stimulated the development of longer roots, which in turn improved the absorption of nitrates and other nutrients.

The plants in treatment T2 likely made better use of the available nitrogen compounds (ammonium ions and nitrates). Although nitrate levels in the T2 tanks were lower than those measured in T1, the elevated concentrations of ammonium ions (NH₄⁺) may have contributed to faster growth, as plant development depends on the balance between NH₄⁺ and NO₃⁻ [41]. The greater root growth observed in T2 plants may be associated with a more favorable NH₄⁺–NO₃⁻ ratio, which could have stimulated root biomass formation. The increased root length in T2 indicates that plants had improved access to water and nutrients, supporting the development of a more extensive root system. The higher plant weight in T2 reflects better biomass accumulation, which is also linked to the efficient uptake of other macro- and micronutrients besides nitrogen.

5. Conclusions

This study demonstrates that feeding time is a critical management factor in carp–lettuce aquaponic systems. A later feeding schedule (T2: 11:00, 14:00, 17:00) improved carp growth and feed efficiency, supported higher lettuce biomass, and maintained suitable water quality, indicating more efficient nutrient use rather than increased nutrient loading. These results suggest that synchronizing feed delivery with periods of higher plant nutrient demand can enhance nutrient cycling and overall system productivity. Feeding time optimization is therefore a low-cost, easily adjustable tool that should be considered alongside feeding rate, frequency, and stocking density when developing management protocols for sustainable aquaponic production.