Ulva rigida C. Ag. was cultivated in 750 1 tanks at different densities and flow rates with waste waters from Sparus aurata cultivation systems, to estimate the conditions for optimizing biofiltering efficiency and yield. To maximize both characteristics: i) seaweed density should be adjusted to 2.7 g FW l ^-1 (1.2 kg m^-2) and ii) NH₄⁺-nitrogen flow should be (at least) 150 mmo1 m^-2 d^-l. Such conditions will yield 27 g DW m^-2 d^-l with uptake efficiencies of 76 % of the daily input in our system at 12 volumes d^-l (110 mmo1 m^-2 d^-l) at seaweed : fish biomass ratio of 1:4. NH₄⁺ removal efficiencies would probably be increased if fish were feed as early as possible in the morning to provide maximum NH₄⁺ concentrations during the light period, as uptake efficiencies are higher than in the dark.