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JOURNAL OFAPPLIFD AQUACULTURE INTRODUCTION The carrying capacity of aquaculture systems employing water reuse is limited by the accumutation of ammonia, the principal ni- trogenous excretory waste of cultured fish and invertebrates. Am- monia exists in a pH- and temperature-governed equilibrium be- tween atoxic un-ionized form (NNE) and a relatively innocuous ionized form (NH4 } (Piper et al. 1983). Ion exchange can be used to remove ammonia from freshwater fish culture systems, but biofiltration is more commonly employed for this purpose (Wheaton 1977; Spotte 1979). Biofittration is the conversion of ammonia to relatively innocuous nitrate by a filter that contains nitrifying bacteria that are attached to supporting me- dia such as gravel, plastic rings, oyster shells, or other materials with a large surface area. Nitrification is accomplished primarily by two genera of aerobic, chemosynthetic bacteria: Nitrosomonas spp., which oxidizes ammonia to nitrite, and Nitrobacter spp., which oxi- dizes nitrite to nitrate. Other organisms also occur in the biofilter bed, including heterotrophic bacteria that oxidize carbonaceous or- ganic wastes (Spotte 1979). There are numerous biofilter designs, including submerged (up- flow, downflow, and horizontal), trickling, and rotating disk or drum (Wheaton 1977; Kaiser and Wheaton 1983; Miller and Libey 1985; Palter and Lewis 1988). Submerged biofilters have the ad- vantage of being mechanically simple and compatible with a wide range of flow rates and media types. Recently, submerged biofilters have been operated with fluidized filter beds, resulting in large in- creases in carrying capacity and efficiency over conventional biofil- ter designs (Palter and Lewis 1988; Jewell and Cummings 1989). Previous research has demonstrated that the fixed biofilm (i.e., biofilm attached to a substrate) nitrification process conforms to the half-order/zero-order kinetic model introduced by Haremoes (1978). Half-order kinetics (ammonia removal dependent upon, ammonia concentration) occur at relatively low ammonia concentrations be- cause of incomplete ammonia penetration through the biofilm. Zero- order kinetics (ammonia removal independent of ammonia concen- tration) occur at relatively high ammonia concentrations as a result of insufficient oxygen diffusion through the biofilm or other limita-