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14 JOURNAL OF APPL IED AQUACULT'URE becomes the ammonia production rate of the food (g NH3-N/kg food) rather than the ammonia production rate per kilogram of fish biomass, and B becomes the carrying capacity feeding rate (kilo- grams/day) rather than the carrying capacity fish biomass. The nu- merator of Equation I2 should be multiplied by 1.44 (to account for the conversion of milligrams to grams and minutes to days) if this equation is solved in terms of feeding rate rather than fish biomass. Instead of predicting the fish biomass that will be held by a biofil- ter of known volume, it is often desirable to predic! the biofilter volume required to support a known biomass of fish. Substituting V for (XSD), Vr for (XrSrDr), and rearranging terms in Equation 12, the following equation is obtained: lOgip(1 - BA-LC FCao ~ VrF (13) V= / / logt0 (1-I O.11T-0.20 \ Rl Fr `\ \ O.i1TR-0.20 J J Equation 13 is solvable only if FC > (BA - LC). That is, equi- librium cannot occur unless the rate at which ammonia is brought to the biofilter (which is equal to the ammonia concentration times the flow rate) exceeds the rate of ammonia production (which is equal to the fish biomass times the ammonia praduction per unit of bio- mass minus the ammonia removed by the flush). Thus, flow rate establishes a ceiling on the carrying capacity of any biofiltration system, regardless of the size of the biofilter. Assumptions The theoretical basis of the model and experience with its appli- cation indicate that several assumptions must be met to ensure accu- rate predictions of carrying capacity. These assumptions are: 1. Dissolved oxygen concentration and pH are not limiting nitri- fication (pH > 6.5 and dissolved oxygen concentration > 5.0 mg/I throughout the biofilter). 2. Ammonia removal by the biofilter is equal to ammonia pro- duction by the fish. That is, ammonia concentrations must be