In this study, water quality and mass balances in the closed recirculating culture of Japanese flounder were examined. The closed system consisted of rearing thank, foam-separation and nitrification units, pH and temperature controlling units, and a circulation pump. Fish were fed commercial pellet diet for 90 days.
Average values of oxygen saturation and turbidity in the rearing water through the culture were 96% and 3.1 TU, respectively. Ammonia was mostly kept at less than 1 mg-N/l, and nitrate increased linearly according to the amount of diet supplied. It is estimated that 33% of dietary carbon （C） was accumulated in the fish, as well as 52% of nitrogen （N）, and 26% of phosphorus （P）. The carbon released as CO2 was 63%. Nitrate in the rearing water corresponded to 28% of N-input, and 66% of P was settled in the nitrification unit as precipitate.