10min. Alkalinity was supplied in form of sodium bicarbonate(NaHcou) to maintain pH above 6, whenever necessary Additionally to the 9 experimental tanks, HIGH accumulation hical Com- each flow rate of23LImin) ersity. The RAS contained 4 culture tanks(0,450 m3 stocked with group-housed Nile tilapia 184 gi 155 indlm')iThese 08014b. culture tanks had the objective to load up this system and there fore contribute to the release of fish-related substances. In addition these fish were fed a high amount(mean feed intake: 0,73 kg/day: source of feed-related substances) of the feed described earlier all male. through a 24-h automatic belt feeder, This system also contained ican cat up-flow sludge blanket nds) and a 5-year-old mature biofilter and an denitrification reactor(0.48 m3) both functioning a the potential ds) were source of system-related substanc Wagenin Experimental fish(Nile tilapia, African catfish and European(6 tanks eel) in the HIGH accumulation RAS received the same water as ties and the culture tanks containing Nile tilapia while experimental fish in imental the Low accumulation RAS did not(no culture tanks were used) change Low accumulation RAS was operated at high water exchange ra nks per(1500 LIkg feed/d) with a young biofilter(in use for one month and started fully acclimated based on TA-N <0.3 mg/L and N mgll. water concentration during the adaptation period) and had no 30 L/kg culture tanks. Therefore, it could be considered as a proxy for flow- through systems Daily water exchange was performed from sump 1(Fig. 1), The tilapia water exchange volume(L) was calculated based onthe feed load of carbo- edure the previous day and the exchange rate of each RAS: HIGH accumu lation RAS(30 L/kg feed/day) or Low accumulation RAS(1500 LIkg at Was removed tinu- feed/day). The total volume of water to be replaced was d feed everyday from the system which had its inlet tap closed. Only after the water removal was completed the new well water supply was allowed in the system,
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