A water reuse system incorporating ozonation and a fluidized-bed, granular carbon biofilter supported approximately six times more fish than a conventional system of identical size and flow rate that incorporated oxygenation and a fixed-bed gravel biofilter. Increasing the hydraulic loading rate to 44 ML/cm2·s in an upflow granular carbon biofilter caused the carbon particles to become suspended in upflow currents (fluidized) and tripled ammonia removal. Ozonating the water prior to sedimentation, physical filtration, and biofiltration also increased ammonia removal, by reducing organic matter concentrations and favoring the growth of nitrifying bacteria over heterotrophic bacteria in the biofilter. Ozonation directly reduced nitrite concentrations by oxidation, and it reduced turbidity. Ammonia removal per unit volume was a more useful criterion for evaluating fluidized-bed biofilters than ammonia removal per unit of substrate surface area.