4.1. Abiotic Factors
In the present study, the co-cultivation of red tilapia and rockets was performed in an aquaponic system for 30 days. The supplementation of Fe alone and its combination with K was examined in relation to no external inputs treatment (control). The effects of the various treatments on water quality indicators, fish growth performance, fish histopathology alteration markers, Fe and K loading in fish gills, and plant growth parameters were assessed.
In an aquaponic system, the water temperature setting is dependent on the fish and plant species. Tilapia can live in waters with a wide range of temperature, 17–35 °C, with the optimum temperature range for normal development, reproduction, and growth to be about 25–32 °C, depending on the fish species, size, and genetic variation [
28,
29,
30,
31,
32,
33]. In the present study, the water temperature was kept at 22.8–22.9 °C, meeting the requirements of both tilapia and rocket plants. pH management is also important in an aquaponics system. In hydroponics, the pH is generally set between 4.5 and 6. In RAS systems, the pH is between 7.0 and 8.0 in order to meet the requirements for both fish and nitrifying bacteria. The pH of aquaponics systems is a compromise between the above-mentioned diverse requirements of its living components; the optimal pH range appears to be 6.0–7.5 [
34]. pH higher than 7.5 causes reduced micronutrient and phosphorus solubility; thus, plant uptake of certain nutrients is restricted [
34,
35]. Optimum availability for K occurs at pH range 6–8, while the optimum availability for Fe is at pH range 4–6 [
34]. The pH in our study was kept almost constant over time (
Figure 1) for all the treatments. The mean values were 7.49–7.54. Temperature and pH values in aquaponic systems are important parameters for plant needs/nutrition and for fish welfare.
DO is very important for the fish, plant roots, and bacteria. Plant roots require lower DO values than fish. Most of the fish need DO 5 mg/L or more, while tilapia can tolerate very low DO concentrations, i.e., 1.0–1.5 mg/L [
36]. A general rule is 6 mg/L for cold-water fish and 4 mg/L for warm water fish [
37]. Plant roots and nitrifying bacteria require at least 3 mg/L DO [
34]. So, in an aquaponic system, if the DO is set to meet the fish requirements, the requirements for plants and bacteria are also met. In the present study, the oxygen levels were 8.41 ± 0.042, 8.36 ± 0.032 and 8.32 ± 0.036 for the control, Fe, and Fe + K treatments, respectively.
EC appeared higher at the Fe + K treatment, where fertilization occurred. EC indicates the number of charged ions circulating in the water column, so the more ions (nutrients) circulating, the higher the value of conductivity [
38,
39].
In an aquaponic system, the fish feed is the main nutrient source [
40]. Fish metabolize the feed, and their feces along with the uneaten food, become the nutrient source for plants through oxidation of the produced ammonia to nitrate by the nitrifying bacteria. It has been suggested that 80% of the required nutrients for plants can be provided by the fish feed [
41]. The rest 20% concern nutrients that are absent or in low levels in fish feed, such as Ca, K and Fe, and should be supplemented either directly to the water or as foliar fertilization. Nitrogen can be absorbed by plants in two forms, nitrate or ammonium, depending on the concentration and the plant physiology [
42]. In the present study, the low ammonium mean values and the gradual rise of nitrate levels (
Figure 2) proved the efficiency of the filter in oxidizing the produced ammonia. During the 30-days study period, the mean nitrate concentration (mg/L) was 122.56 ± 7.367, 120.82 ± 5.911, 120.32 ± 8.584 for the control, Fe, and Fe + K treatment, respectively. The daily supply of 13.47–14.03 g of fish feed provided efficient nitrogen for plant nutrition. According to Santamaria et al. [
43], leaf number and yield of the rocket are low with NH
4 nutrition, whereas they reach the highest values with the 50:50 NH
4:NO
3 ratio. In their study, water and N-use efficiencies increased in the rocket with the increase in NO
3-N percentage in the nutrient solution. In our work, the mean Fe concentration (mg/L) for the 30-days study period was 3.61 ± 0.338 for Fe treatment and 3.55 ± 0.354 for Fe + K treatment, while the K concentration (mg/L) for the Fe + K treatment was 112.00 ± 9.953. These values are close to the optimum target values of 5 mg/L for Fe and 120 mg/L for K.
4.2. Fish Growth Performance
During the 30-day study period, red tilapias were fed twice daily ad libitum. For fish, FCR is used for the conventional measure of livestock production efficiency, while SGR is connected to fish growth. Lower FCR values and higher SGR values indicate a suitable growth performance. Red tilapia in the aquaponic systems of the present study showed high growth performance as judged by FCR and SGR values, which were similar along the three treatments. Additionally, the supplementation of Fe and K did not affect fish survival. All the above-mentioned results indicate that the addition of Fe or K did not affect fish growth. There are few articles examining the impact of fertilizers on tilapia growth. Rafiee et al. [
44], working in aquaponics with a combination of lettuce and red tilapia, reported different fish survival rates between the treatment of no inputs and the treatment of Fe, K, Ca, Mg, Mn, P, and Zn addition (58% and 73%, respectively). Additionally, the latter treatment resulted in higher FCR and lower SGR compared to the treatment of no external nutrients. Ru et al. [
33] reported 31.5% increased feed consumption and 14.3% higher FCR of Nile tilapia reared in an aquaponic system where nutrient supplementation occurred. The growth performance indices of the present study (survival, FCR, and SGR) indicate better fish growth compared to the two above-mentioned works [
44,
45]. Moreover, the absence of differences between treatments in all other measured fish growth parameters, such as weight gain, final weight, and length, highlights the neutral effect of Fe and K supplementation on fish development.
4.3. Fish Histopathology
Histological studies of farmed fish in aquaponics systems are limited. Vlahos et al. [
16] performed the histopathological examination in sea breams in a brackish water aquaponic system, while Nozzi et al. [
45] examined the histopathology of sea bass in a freshwater aquaponic system. Nevertheless, the experimental design of these studies did not include nutrient supplementation.
Fat deposition in the fish liver is affected by the dietary lipid content. Lower dietary lipid content results in lower liver lipid content and smaller and lesser lipid droplets [
46]. In the present study, red tilapia were fed ad libidum twice daily with a fish feed containing 6.5% crude fat. Minimal lipid accumulation was detected at the liver of control and Fe treatment and mild lipid accumulation to the Fe + K treatment. No steatosis sign was detected in any of the treatments. Liver histopathology of the Fe + K treatment also showed the presence of granulomas, regions with larger nuclei, necrotic regions, and regions with haemmoradge. These alterations, by the frequency of their appearance in the examined fish, were characterized as mild. Since the liver is an important store of energy reserves, where dissection is possible, the hepatosomatic index (HIS) is often used as an estimate of the energy status of the fish [
47,
48]. According to Singh and Srivastava [
49] (2017), a decrease in HIS has a relation to toxicity on fish. Values between 1.3% and 1.97% have been reported for red tilapia (
Oreochromis spp.) [
50,
51,
52,
53]. Our values of HIS are in agreement with these studies, indicating no toxicity. Midgut had a normal structure in all three treatments. Individual lesions were detected in the midgut in 2 of the 15 examined fish and were characterized as random and non-specific.
Gills are the main respiratory organ of fish. Changes in dissolved oxygen and water temperature can lead to the rapid and reversible morphological change of the gills. Several species such as
Oncorhynchus mykiss,
Rutilus rutilus,
Perca fluviatilis,
Anguilla anguilla,
Ambloplites rupestris,
Micropterus salmoides, and
Carassius carassius have been reported to be able to reduce their respiratory area and their oxygen demands as well [
54,
55,
56,
57,
58,
59]. Red tilapia is a warm water fish and can tolerate low dissolved oxygen values of about 4 mg/L [
37]. The primary lamellae hyperplasia observed at the present study in all treatments may be a physiological result as the dissolved oxygen was between 8.32 and 8.41 mg/L, higher than the tilapia’s lower demands. Yavuzcan Yildiz et al. [
8] reported that in an aquaponic system, the high level of suspended solids could provoke damage to the gill structure, such as epithelial detachment, hyperplasia, lamella fusion, and reduction in epithelial volume. In the present study, the histopathological examination of the gills revealed similar results as Yavuzcan Yildiz et al. [
8] described. Uneaten food and feces were daily siphoned; however, a breakdown in small particles still occurred. These particles are very difficult to collect and are potentially dangerous.
In general, the very limited cases of mild histopathological alterations in liver, midgut, and gills and, moreover, their complete absence in the majority of fish indicate that tilapias of our study were well adapted to the aquaponics system.
4.4. Gills EDX Analysis
Freshwater fish can absorb waterborne metals through their gill epithelia due to the binding capacity of the mucous layer that covers the gill arches and their close contact with the surrounding environment [
60,
61]. Gills absorb active ions; thus, their oxygen absorption and osmoregulation capability can be affected, leading to numerous histopathological changes in the gill [
62]. Wepener et al. [
63] reported that when
Tilapia sparrmanii was exposed to 319 mg/L iron concentration, the iron in gill tissue increased significantly after only 2 h exposure, remained elevated during the 72 h exposure period, and returned to control values after 96 h of exposure. In our research, red tilapias were exposed to significantly lower iron concentrations (3.61 mg/L and 3.55 mg/L for the Fe and Fe + K treatment, respectively), and no iron accumulation was detected at the gills after 30 days.
High potassium concentrations disrupt various physiological functions of fish, such as osmoregulation, acid-base balance, muscle contraction, and nerve function [
64,
65,
66]. According to Mount et al. [
67], a 305.6 mg/L potassium concentration could be lethal for fathead minnow. Davidson et al. [
68] noticed gill irritation at concentrations of 110–130 mg/L K. In our research, the mean K concentration in the water was 112.00 mg/L for the Fe + K treatment, while for the control and Fe treatments were 3.81 mg/L and 4.44 mg/L, respectively. This fact was well reflected in the K accumulation in gills of the Fe + K treatment fish, where the K detected by EDX was seven times higher than the other two treatments. This high accumulation did not seem to affect the gill histomorphology, as mild histopathological alterations were detected in all three treatments. No mortalities were observed, indicating that the K concentration was not lethal.
4.5. Plant Growth Performance
Leaves fresh weight of the rocket plants that received additional Fe during their cultivation outweigh the plants of the other two treatments; however, the differences between this group and Fe + K group were not statistically significant. Control plants showed inferior growth in terms of fresh weight of the aerial parts, but also in the produced biomass per unit area, which is the measure of the system productivity. Nutrient amendments have been reported to enhance the productivity of the aquaponic systems, which are inherently deficient in Fe and K [
33]. Nevertheless, it is important to examine and target the minimal possible perturbations of the system, hence the minimal inputs that sustain the productivity without compromising the sustainability of aquaponics. It was proved in our experimental setup that the addition of only Fe is adequate to succeed in optimum growth. K addition seems to be not indispensable for rocket growth. Considering the fact that K seems to accumulate to the gills, and maybe this accumulation can have long-term effects on fish health, we would suggest that in aquaponics systems with rocket cultivation, no extra K addition is needed.
There are a few articles working with a rocket in aquaponics, but their experimental questions are far from the target of the present study. Indeed, the educational use of aquaponics system [
69] and life cycle assessment modeling [
70] necessitated the use of rocket plants in mixture with many other crops; thus, the results could not be compared with ours. Barbosa et al. [
71] examined rocket growth under two different system water volumes, and their results indicated poor growth performance reaching 8–15 times lower leaves fresh weight compared to ours, mainly due to extremely high sowing density of seedlings. Finally, Lennard and Ward [
72] concluded that rocket does not adapt to aquaponic culture as well as other herbs, because they found increased plant growth in the hydroponics system. Our results do not corroborate these findings since the growth of rocket across all treatments in the present study showed 2–3 times higher values compared with both hydroponics and aquaponics as reported in the experiment of Lennard and Ward [
72].