Differential equations were used to model the efficacy of UV radiation in three hypothetical aquaculture systems: (a) a plug-flow system, (b) an idealized closed system with no influx of pathogens, and (c) a conventional closed system in which the influx of pathogens is continuous. The equations demonstrate that, in a conventional closed system, the mass of pathogens never reaches equilibrium with the rate of reduction, even if the UV sterilizer is 100% effective. This suggests that agents such as UV radiation, which do not form persistent residuals, are incapable of preventing the spread of water-borne pathogens in closed systems. Use of UV radiation in aquaculture should therefore be limited to sterilization of raw water supplies and discharges into receiving waters, both of which are single-pass applications.