RAS were kept undisturbed (no sampling procedures) during 57 days of growth period. Daily water exchange was performed according to a standard procedure consisting of calculating the water exchange based on the feed load of the previous day and refreshing 30 L/kg feed/day (HIGH), 70 L/kg feed/day (MIDDLE) or 1500 L/kg feed/day (LOW) from sump 1 (see Fig. 1; conductivity and alkalinity of supply water were 189 μS/cm and 72.5 mg CaCO3/L, respectively). The total volume of water to be replaced was removed every day fromthe system which had its inlet tap closed. Only after the water removal was completed the new well water was allowed in the system. Alkalinity was never supplied to the systems throughout the experimental period. No water was lost from the systems except for water loss due to evaporation that was topped up before daily water exchange.