## 4 Discussion
The concentrations of nutrients are typically lower in aquaponics water compared to standard hydroponics solution, but most leafy vegetables can grow at lower concentrations than in standard hydroponic solution [56]. However, most plants require nutrient supplementation in aquaponics to deal with their nutritional requirements [56]. The nutrient requirements of plants vary with developmental stage, environmental conditions, plant species, and variety [12; 56]. In the present study, we added micronutrients (Fe, Cu, Mn, Zn, B) to the aquaponic systems to comply with the plant's growth requirements. The modified nutrient solution was prepared by considering safe nutrient limits for the plants and rainbow trout [28; 57]. The amount of Fe, Zn, and B were higher in spinach leaves grown in aquaponics than in hydroponics, most likely due to the addition of micronutrient solution and spraying with this solution. The amounts of Ca and Mg were also higher in the spinach grown in the aquaponics treatment, which can be linked to the Ca and Mg in the fish feed. The amount of macronutrients N, P, S, and K were higher in hydroponically grown spinach which was due to the added nutrients into the hydroponic system in the form of the solution and spray. According to the manufacturer, the Substral solution contained nitrogen (N) 6%, phosphorus (P) 1.3%, potassium (K) 5%, sulphur (S) 0.6% and chloride (Cl) less than 0.5%. The concentrations of macronutrients in hydroponics nutrient solution explain the higher amounts of macronutrients in spinach grown in hydroponics. In our study, the concentrations of macronutrients in spinach shoots were comparable to those reported by [PERSON] et al. [58], except that magnesium was lower in spinach grown in hydroponics and calcium higher in aquaponics. As to micronutrients, the concentration of iron was higher (523.3 mg/kg \(\pm\) 75.05) in spinach grown in aquaponics compared to the reported 267 mg/kg [59] for spinach purchased from the vegetable market. The concentration of iron can vary in different parts of plants. In general, iron concentrations in leaves in most plants range between 0.1 to 5000 mg/kg [60]. The concentration of zinc was lower in aquaponics (526.6 mg/kg \(\pm\) 142.9) and hydroponics (206.6 mg/kg \(\pm\) 55.7) compared to the concentration reported in another study for spinach (3230 mg/kg) [59]. Even if the dry weight of spinach at the end of the experiment did not significantly differ statistically between aquaponic and hydroponic treatments, in aquaponics, the total plant weight was 43% higher (40% for shoot, 70% for root) than in hydroponics. The nutritional content in spinach leaves varied and depended upon the quantity of nutrients supplied and present in the circulating water. In a hydroponics system, the addition of Substral nutrient solution, compensation of evaporated water with Substral solution, and daily spraying of plants with this solution provided the nutrients that were essential for the spinach growth while in aquaponics treatment, the plants were getting nutrients with the circulating water. In addition, the micronutrient solution for aquaponics spinach fulfilled the expected needs of micronutrients. The color of spinach was vibrant green with no signs of yellowing. Our results are supported by the earlier studies showing that the aquaponic systems can produce the same or higher yield compared to hydroponics [27; 61; 62]. A study on spinach grown with stellateurgeon (_Acienser stellatus_) in an aquaponics system showed that the growth, quantity, and quality of aquaponically grown spinach were similar to the field-grown spinach [63]. The concentrations of off-flavors were relatively low for GSM (2-8 ng/L) and MIB at moderate levels (15-35 ng/L) in the water of the studied systems. These are typical for a RAS (5-25 ng/L GSM, 50-130 ng/L MIB, [64] and 128 ng/L GSM, 94 ng/L MIB) [23] although each RAS is a unique system.