## 4 Discussion  
### Rearing Condition  
Water quality parameters in all experimental treatments showed a stable fluctuation and were within limits reported by other researchers [7; 8; 12; 17]. The results of the present study in terms of TANin and TANout show a range of values for all treatments ranging from 0.4 to 0.68 mg/L and 0.4-0.7 mg/L, respectively, without showing statistically significant differences. These values are higher than those reported by [PERSON] et al. [7] in a brackish aquaponic system with gilthead sea bream and rock samphire at 8 ppt and 20 ppt salinities, which ranged from 0.33 to 0.39 mg/L. Additionally, much lower ammonia values (0.05 mg/L) were reported for sea bass in a brackish water aquonics system [17] compared to the present study. In addition, much higher ammonia values than those of the present study were reported for tilapia reared in a freshwater and saltwater aquaponic system, which were 1.87 mg/L and 60.5 mg/L, respectively [44]. Other studies reported average ammonia values (range of 0.02 mg/L) in aquaponic systems with tilapia and various herbivovous species in fresh and brackish water of 1.2 ppt salinity [45], which are much lower than the present experiment. Regarding the nitrate ions, the results show that nitrate ions (NO3\({}^{-}\)) at the inlet checkpoint of the hydroponic tank reported herein (114.61 mg/L for 20 ppt salinity, 122.38 mg/L for 14 ppt salinity, and 120 mg/L at 8 ppt salinity with no significant differences) were higher than those (76.4-77.2 mg/L) reported by [PERSON] et al. [7] in a brackish aquonics system of 20 ppt and 8 ppt salinities with gilthead sea bream and rock samphire. Concerning the nitrate ions at the water outlet from the hydroponic tanks, the results show a range of variation (109.76 \(\pm\) 16.09 mg/L for 8 ppt salinity, 115.41 \(\pm\) 14.73 mg/L for 20 ppt salinity, and 111.41 \(\pm\) 17.83 mg/L for 14 ppt salinity) which was higher than those reported by [PERSON] [46] for sea bass and rock samphire in an aquonic system with 20 ppt, 14 ppt, and 8 ppt, respectively (8 ppt: 69.19 \(\pm\) 6.81 mg/L, 14 ppt: 69.50 \(\pm\) 7.35 mg/L, and 20 ppt: 70.81 \(\pm\) 7.47 mg/L). Generally, it is known that high nitrate ions may indicate the aquaponic system's proper functioning and increase the filter's oxidative capacity [7; 12; 47; 48]. Regarding the nitrite ions, the findings herein show no significant differences in both treatments (ANOVA, \(p\) > 0.05). The results from the present study are comparable to that of previous studies [17] reporting similar values of nitrite ions (0.1 mg/L) in a brackish aquaponics system co-cultivating sea bass and _Beta vulgaris_ var. _cicla_. Previous own studies [7] reported a more comprehensive range of variations in nitrite ion concentration between 0.82 and 0.89 mg/L in a brackish aquonic system with co-cultivated gilthead sea bream and rock samphire under salinities 20 ppt and 8 ppt. The analysis herein indicated no statistical differences in PO4\({}^{=}_{\text{in}}\) and PO4\({}^{=}_{\text{out}}\) in both treatments (ANOVA, \(p\) > 0.05), and these were lower compared to those reported by [PERSON] et al. [8] in an aquaponic system with seagrass and lettuce. In general, the PO4\({}^{=}\) concentrations are expected to range between 3 and 52 mg/L [49; 50]. In this study, the pH in fish tanks (pH\({}_{\text{FT}}\)) varied between 6.0 and 6.5, while the pH\({}_{\text{GB}}\) in grow beds varied between 6.3 and 6.6. These values were lower than those previously reported by [PERSON] et al. [7] (7.54 to 7.73) or by [PERSON] [46] (6.88 to 7.08) in brackish aquaponics systems. It has been shown that pH in aquaponic systems is one of the main factors affecting fish metabolism and microbial activities, thus influencing the nitrogen supply [51; 52; 53]. Furthermore, it is known that pH also regulates the solubility of micronutrients such as calcium, phosphorus, potassium, magnesium, etc., and affects the bioavailability of nutrients for uptake by plants [54].