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Pa!!er flow rate through the biofilter. This measurement was converted to the ammonia removal rate pier unit of biofilter bed volume (m~). Measurements of the proportion of empty space within the media (i.e., voids fraction) and volumes of the biofilters were used to estimate the amount of water held by each biofilter. The latter quan- tity was divided by the water flow rate through the biofilter to esti- mate the retention time in each biofilter. RESULTIS AND DISCUSSION Etifects of Ammonia Concentration on Ammonia Removal Ammonia removal kinetics were investigated in four biofiltration systems: fluidized granular carbon +ozonation, fluidized granular carbon + oxygenation, granular carbon +ozonation, and gravel + ozonation. Data from the gravel and gravel + oxygenation biofiltra- tion systems did not encompass a wide enough range of ammonia concentrations to permit similar investigations. Ammonia removal was strongly dependent upon the ammonia concentration when the ammonia concentration vvas below 2.0 mg/l, as indicated by coeffi- cients of determination I;RZ) that ranged from 0.62 to 0.88 for the four biofilters (Figure 2). At higher ammonia concentrations, am- monia removal varied sc-mewhat but was independent of the ammo- nia concentration. Thu;, ammonia removal in these biofiltration systems conformed to tlhe half-order/zero-order kinetics model that was developed by Haremoes (1978) and that was verified for aqua- culture by Bovendeur et al. (1987). Fluctuations in ammonia re- moval at high (> 2.0 nng/i) ammonia concentrations may have re- sulted from decreases in nitrification due to the accumulation of organic matter in the biiofiltration systems when fish loads and feed- ing rates wcre temporarily increased to high levels. Organic loading can stimulate the proliferation of heterotrophic bacteria that com- petewith nitrifying bacteria for space and dissolved oxygen (Siddall 1974; Parker and Richards 1986). Oxygen concentrations in the ef- fluents of the biofiltration systems remained above 7.0 mg/l throughout these studies.