The efficiency of a biological filter can be measured by determining the amount of oxygen consumed by bacteria biomass in the filter. The supply of dissolved oxygen is a principal factor limiting filter performance in high-density aquaculture systems. A mass balance technique is used in this paper to quantify relationships defining the filter volume and flow-through rates required for biological filter design. Filter designs employing flow recirculation with supplemental aeration are the most successful. Filter acclimation by chemical addition and filter performance techniques are also discussed.