Ulva rigida was cultivated in 750 1 tanks at different densities with direct and continuous inflow (at 2, 4, 8 and 12 volumes d_1) of the effluents from a commercial marine fishpond (40 metric tonnes, Tm, of Sparus aurata, water exchange rate of 16 m3 Tm-1) in order to assess the maximum and optimum dissolved inorganic nitrogen (DIN) uptake rate and the annual stability of the 'Ulva tank biofiltering system' . Maximum yields (40 g DW m-2 d-1) were obtained at a density of 2 .5 g FW 1-1 and at a DIN inflow rate of 1 .7 g DIN m-2 d-1 . Maximum DIN uptake rates were obtained during summer (2 .2 g DIN m-2 d-1), and minimum in winter (1 .1 g DIN m-2 d-1) with a yearly average DIN uptake rate of 1 .77 g DIN m-2 d-1 . At yearly average DIN removal efficiency (2 .0 g DIN m-2 d-1, if winter period is excluded), 153 m2 of Ulva tank surface would be needed to recover 100% of the DIN produced by 1 Tm of fish .Abbreviations: DIN= dissolved inorganic nitrogen (NH4 + NO3 + NOz ) ; FW= fresh weight ; DW= dry weight ; PFD= photon flux density ; V= DIN uptake rate