The daily net nitrogen uptake (i.e. nitrogen retained by the fish) was found to be 240 mg N kg fish−1. This is equivalent to a daily uptake of about 1.6 g protein per kg fish, or about 25% of protein normally added with feed. Nitrogen excretion was found to be about twice as high as the net uptake, i.e. gross daily N uptake was about 700 mg N kg fish−1. Bio floc residence time was estimated to be about 8 h, indicative of the high dynamics of the bio floc system. The results of this experimentwere used to evaluate experimental and computational approaches to determining nitrogen fluxes.