Sustainability of Growth Performance, Water Quality, and Productivity of Nile Tilapia-Spinach Affected by Feeding and Fasting Regimes in Nutrient Film Technique-Based Aquaponics

Aquaponics crop production relies heavily on water quality, particularly in recirculating water systems. According to [32], declining water quality metrics have an impact on crop performance, quality, and/or yield, in addition to fish physiology, growth rate, and feed efficiency. Water quality during the experiment stayed within established standards for the rearing of O. niloticus, demonstrating that the water quality management of the culture system was adequate, regardless of feeding and fasting regimes [33,34]. During the experiment, the difference in feeding and fasting regimes of Nile tilapia did not significantly affect the pH, as the range was 7.20 to 8.09, which is generally considered a suitable range for the aquaponic system [35], asplants are considered to have access to nutrients in the pH range of 5.8 to 6.5. According to [36], nitrifying bacteria prefer a pH range of 7.0 to 9.0, whereas fish require a pH range of between 6.5 and 9.0 (Table 1). Dissolved oxygen levels decreased up to 12% as feeding days increased due to the oxygen needs of fish and microbes. Generally, the average DO in all treatments was slightly higher than the average DO level suggested for aquaculture, which is greater than 5 mg/L. Nitrifying bacteria need a DO range of 4–8 mg/L to maximize the nitrification operation [37]. The dissolved oxygen levels were sufficient in the aquaponics system. Many studies have reported that the electrical conductivity (EC) measured between 300 and 1100 s/cm is suitable for aquaponic solutions [36,38]. In a hydroponic solution, the TDS should be between 1000 and 1500 mg/L. Relatively low TDS concentrations (200–400 mg/L) will yield good results because nutrients are continuously created in the aquaponics system [36]. EC (636.33 ± 39.37–654.56 ± 4.68 µs/cm) and TDS (321.02 ± 3.45–361.72 ± 8.07 mg/L) were within the range of favorable conditions (Table 1). The calculated alkalinity was within the permissible range of 5–500 mg/L, and ranged from 35.62 0.26 to 36.87 0.61 (p > 0.05) [39]. In the aquaponics system, nitrification is a biological reaction that converts water from the toxic form (ammonia nitrogen, NH3-N) caused by fish in bio-filtration units to the non-toxic form nitrate (NO3-N), which the plant benefits from. According to [40], nitrite (NO2-N), a nitrification intermediate product, is also widely recognized as being toxic (at low concentrations) to both fish and plants: according to [38], the acceptable limit for ammonia in the aquaponics system is less than 1.0 mg/L. In our research, the level of total ammonia nitrogen increased (p p > 0.05). According to [41], nitrate, produced as a result of nitrification, is the preferred type of nitrogen for plants due to its low toxicity levels. [38] concluded that an aquaponics system has an endurance limit of 150 mg/L for nitrate, whereas [42] found that the level of nitrate in aquaculture water should be 0.1–4.00 mg/L of nitrate. In the current experiment, the nitrate-nitrogen concentration in different feeding and fasting regimes ranged from 0.31 ± 0.02 to 0.37 ± 0.02 mg/L (p > 0.05). According to [43] the plant absorbs phosphorus in the form of ionic orthophosphate, whereas [44] determined that in an unprocessed aquaponic system, the orthophosphate level never rises above 2.0 mg/L but often hovers around 1.0 mg/L. In this experiment, it was found that the level of phosphate decreases significantly as feeding days decrease (0.63 ± 0.03–0.51 ± 0.02 mg/L), as a result of plant absorption. However, the phosphate level was, in all treatments, found to be within the desired range. The findings of the current study established that the calcium level ranged between 37.84 ± 0.57 and 38.35 ± 0.71 mg/L (p > 0.05). According to [45], the acceptable range for free calcium in fish culture water is 25–100 mg/L. A calcium dosage of 27 mg/L was recommended by [46] to enhance fish growth without damaging the spinach yield in the aquaponic system. According to [44], the normal aquaponic nutrition solution contains 12 mg/L of potassium. In this trial, there was no external potassium supplementation, hence the potassium level was lower (11.87 ± 0.41–11.42 ± 0.11 mg/L).