## 4 Discussion  
### 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 \({}^{\circ}\)C, with the optimum temperature range for normal development, reproduction, and growth to be about 25-32 \({}^{\circ}\)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 \({}^{\circ}\)C, meeting the requirements of both tilapia and rocket plants. pH management is also important in an aquaponic 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 nitifying 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 \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Control** & **Fe** & **Fe + K** \\ \hline Leaves fresh weight (g) & 92.54 \(\pm\) 10.816 \({}^{\text{a}}\) & 150.02 \(\pm\) 9.175 \({}^{\text{b}}\) & 125.00 \(\pm\) 8.746 \({}^{\text{b}}\) \\ Produced biomass (g/m\({}^{2}\)) & 1110.44 \(\pm\) 475.032 & 1725.25 \(\pm\) 137.877 & 1500.01 \(\pm\) 307.719 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Rocket growth performance as assessed by the fresh weight of leaves per plant and the marketable produced biomass per unit area. Data are expressed as means \(\pm\) S.E.M (\(n\) = 36 for leaves fresh weight and \(n\) = 3 for total produced biomass). Means in a row followed by different superscript are significantly different (\(p\) > 0.05). \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Control** & **Fe** & **Fe + K** \\ \hline K (At\%) & 4.81 \(\pm\) 0.787 \({}^{\text{a}}\) & 3.85 \(\pm\) 0.538 \({}^{\text{a}}\) & 27.96 \(\pm\) 2.1627 \({}^{\text{b}}\) \\ \hline Fe (At\%) & N.D. & N.D. & N.D. \\ \hline \hline \end{tabular}
\end{table} 
Table 5: K and Fe accumulation in fish gills of all groups presented as At%. Data are expressed as means \(\pm\) S.E.M (\(n\) = 15). Means in a row followed by different superscript are significantly different (\(p\) > 0.05). 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 \(\pm\) 0.042, 8.36 \(\pm\) 0.032 and 8.32 \(\pm\) 0.036 for the control, Fe, and Fe + K treatments, respectively. EC appeared higher at the Fe + K treatment, where fertilization occurred.