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16 JOURNAL OF APPLIED AQUACUI,TURE Model Verification The ability of Equation 12 to predict the maximum carrying ca- pacity of biofilters was verified by comparing the predicted and empirically determined carrying capacities of the gravel, gravel + oxygenation, gravel +ozonation, granular carbon + oxygenation (fluidized bed), and granular carbon +ozonation (fluidized bed) biofilters. The value of C used in these comparisons was 2.0 mg/I. The value of R for each biofilter was calculated from ammonia measurements made when the fish loads were 50% or less of the maximum carrying capacity, Empirically determined carrying capacities for the five biofilters ranged from 7.5 to 69.4 kg; predicted carrying capacities ranged from 5.6 to 81.0 kg. The correlation (r = 0.997, RZ = 0.994, P<0.05) between the empirically determined and predicted carry- ing capacities is near unity, which indicates that the model accu- rately estimated the carrying capacities of the systems (Figure 3). However, with the exception of the fluidized granular carbon + ozonation biofilter, the predicted carrying capacities fell slightly short (by 1.8 to 4.7 kg) of the empirically determined carrying ca- pacities, possibly reflecting the removal of ammonia from the fish culture system by bacteria located outside the biofilter (i.e., free floating in the water, attached to suspended solids, on the insides of pipes, in sedimentation tanks, etc.). This type of error will not be significant if most of the ammonia removal occurs in the biofilter, but it may be important if ammonia removal outside the biofilter is a substantial proportion of the total ammonia removal. Unlike the other biofilters, the carrying capacity of the fluidized granular carbon +ozonation biofilter was over- estimated (by 11.6 kg). This may have resulted from the accumula- tion of organic matter at high fish loads with attendant reductions in nitrification (see Bovendeur et al. 1990) which prevented the theo- retical maximum nitrification rate at 2.0 mg/t ammonia-nitrogen from being realized. Equation 12 can be used to predict biofilter carrying capacity at different flow rates. This capability was verified by comparing the predicted ammonia removal rates and the empirically determined ammonia removal rates of two experimental biofiltration systems.