Recirculating aquaculture system applications of oxygen absorption equipment require consideration of the combined effects of the system's physical, chemical and biological components. Interactions of this type were modeled within a recirculating system incorporating a mixed-flow type rearing vessel, a multi-tube clarifier, a rotating biological contactor (RBC) and a U-tube oxygenator. Finite difference mass transfer calculations, based on reactor theory, were used to predict steady-state dissolved gas levels in component effluents given system operating conditions. The model was calibrated and its predictions verified with data obtained from a pilot scale system of 14 m³ capacity: errors in calibrated model predictions (N = 45) average −1·2 mg l⁻¹ (range −4·0 to 0·1 mg l⁻¹). Model use indicated oxygen transfer costs are reduced 48% through recycle of U-tube off-gas. Further savings are provided by increasing the water recirculation rate from 250 to 350 l min⁻¹ with low to moderate fish feed rates and by regulating oxygen injection based on diel variations in fish respiration. Increasing the gas transfer coefficient (K_La) of the RBC reduced oxygen transfer costs despite resultant elevations in dissolved nitrogen and argon concentrations. Carbon dioxide stripping across the RBC was substantial, varied with K_La, and increased with water recirculation rates.