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10URNAL OF APPLIED AQUACULTURE ozonation (fluidized bed). Carrying capacity was defined as the greatest fish biomass that could be supported by each biofilter with- out the development of total ammonia-nitrogen concentrations that averaged > 2.0 mg/l. Fish were taken from a holding tank receiving a continual flow of fresh water and gradually added to each biofil- tration system over atwo- to four-week period until the carrying capacity was attained. The carrying capacity biomasses were then maintained for at least two more weeks. The fish used to load the biofiltration systems were common carp, Cyp~inus carpio, weighing 200-800 g. They were fed a com- mercial floating pelleted trout diet containing no less than 37.5% protein once daily while in the holding tank and in the experimental biofiltration systems. Because the intent was to assess biofilter per- formance rather than the performance of the cultured organisms, fish growth rates were not measured during these studies, and feed- ing rates were maintained at a constant level equal to 1% of the body weight at initial stocking per day. The fish appeared healthy throughout these experiments, consumed all food offered to them, and suffered no mortalities. Water quality was measured at the entrance and exit points of the biofilters at one- to six-day intervals during all studies. Water qual- ity was also measured less frequently (one- to two-week intervals) at other points in the biofiltration systems (fish tank, sedimentation tank influent and effluent, mixing column influent and effluent, and physical filter effluent). Total ammonia-nitrogen concentrations were measured with an ion-specific electrode. Other water quality variables measured during this study include: dissolved oxygen concentration (polarograph); pH (pH meter};nitrite-nitrogen and ni- trate-nitrogen (Hach Company kits, Loveland, Colorado); turbidity (turbidimeter); ozone concentrations in the feed gas entering the mixing columns (iodometric titration, APHA et al. (1976)); residual ozone (ozone remaining in the water after treatment; amperometric titration, ICinman (1975)}; temperature. Ammonia concentrations and other water quality variables were usually measured 19-23 hours after the daily feeding. The ammonia removal rate of each experimental biofilter was calculated by multiplying the difference between the biofilter influ- ent and biofilter effluent ammonia concentrations times the water