2.1 Experimental design The experiment was carried out for 180 days in the Aquaponics Laboratory, Faculty of Fisheries, Bangladesh Agricultural University (BAU), Mymensingh, Bangladesh. Nutrient Film Technique (NFT) was followed to observe the growth of water spinach in aquaponics system by using two different foliar sprays namely molasses and compost tea. The whole set up comprised a fish holding tank (750L) and three plastic pipes (20 ft long and 4 inch diameter), each with seven separate holes (3 inch diameter).
2.2 Rearing of fish in the tank At first the plastic tank was disinfected with lime and bleach, bottom was prepared with a layer (6 inch) of small gravels; inlet and outlet of the tank for influent and effluent were made with plastic pipes and finally filled with underground water. Preparation of tank included cutting of upper end, washing, liming, plumbing pipes; setting gravels and fills up the tank with water. After disinfection with potassium permanganate solution (2 mg/l for 4 to 5 hours) and acclimatization for 30 minutes, eighty healthy monosex tilapia (O. nilotica) fingerlings (initial length and weight: 9.06±1.22cm and 17.27±6.50g respectively) were stocked in the tank. The fish were fed with commercial floating feed (size-1to3 mm) twice daily at a rate of 5% of fish body weight during 1st and 2nd month and then reduced to 3% in 3rd and 4th month, which finally became 2% in 5th and 6th month.
2.3 Planting water spinach saplings Initially, water spinach seeds were germinated and planted in a plastic tray containing prepared soil. After 20 days, rooted water spinach were lifted up from the soil and then transferred to the plastic glasses. Fish tank waste water was used for watering the plants through a small PVC pipe (influent). This water when passed through the root systems of water spinach, was denitrified by the available nitrifying bacteria and collected to the fish tank through another small PVC pipe (effluent) as clean water for fish. Three pieces of foam were kept on the mouth of each plastic pipe for screening purpose. Foams were washed with clean water daily 2-3 times to remove the debris from it. Watering pipe was also cleaned regularly to prevent clogging and ensure maximum water supply. Water supply to the plant was stopped at night time. No chemical fertilizer or any other additive was used in the spinach bed.
2.4 Preparation of foliar spray (molasses and compost tea) To prepare the molasses spray, 1L of water and 1 table spoon of molasses were taken into a bottle and then mixed thoroughly. Then the solution was taken into the spray machine and sprayed over the water spinach leaf of treatment1 (T1). On the other hand, Compost tea is a solution of manure. For preparation, 1kg compost manure and 2 tablespoons of molasses were mixed thoroughly with 10L of water in a container, then sieved and aerated for 24 hours. The prepared compost tea solution was then sprayed over the water spinach of the treatment-2 (T2) with a spray machine. Foliar sprays (molasses and compost tea solution) were applied at- alternate week. 2.5 Sampling of Fish and plant Fish of the tank and water spinach plant of the grow bed were sampled fortnightly whereas partial harvest of spinach was done monthly. Growth performance of the fish was evaluated by using standard mathematical procedures for analyzing total length and body weight data taken at each sampling. During each sampling, total length and weight of water spinach were taken by deducting the plastic glass weight and during each partial harvest weight of leaves and root were measured.
2.6 Physico-chemical parameters of the fish tank water The physico-chemical parameters of tank water were measured to assess the quality of water before entering plant grow bed (influent) and after passing it (effluent). Dissolved oxygen (DO), Temperature and pH were measured every 15 days interval using test kits. Total nitrogen (Total-N), Electric conductivity (Ec), Carbonate (CO3) Hydrogen carbonate (HCO3), Potassium (K), Sulphur (S), Sodium (Na) and Calcium (Ca) were measured three times at one month interval during the experiment in the “Humboldt Soil Testing Laboratory” at Soil Science Department, Bangladesh Agricultural University (BAU), Mymensingh. 2.7 Data processing and analysis Collected data were analyzed using “Microsoft Office Excel 2007” and “MSTAT” software to determine statistical significance.