Mass budget of the sea bass (Dicentrarchus labrax) farming losses was examined. Experiments were carried out in April, September and November 2001 in tanks of 8.5 m3 containing sea bass of 1, 31 and 53 g, respectively. Samples were taken at the input and discharge points of the tank at hourly intervals over 24 h. They were later analysed for nutrient concentration, as well as particulate organic carbon and nitrogen, chlorophyll a (Chl a) and total bacterial abundance. Despite the high water supply (exchange rate: 50% h−1), NH4 concentration was significantly higher in discharge water than in input water in all three experiments independent of fish size. The same was found to hold true for PO4 for part of the day (April, November) or for the entire day (September). NO3 and SiO2 ions did not show any differences during April. Particulate organic carbon (POC) and nitrogen (PON) were higher in the discharge water in all seasons. Chl a concentration was higher in the discharge water during the entire day in September whereas bacteria presented higher counts in the output water only during part of the day in April. It was found that 5−7% of the nitrogen supplied is released into the water column as fine particulate material in the form of PON and 21−29% as NH4. A small proportion (13−16%) of supplied phosphorus (P) was released as PO4. The results provided in the present paper are useful in assessing environmental changes in water quality in the vicinity of fish farms in the Mediterranean.