Biological filters are essential parts of recirculating aquaculture systems that transform toxic fish compounds such as ammonium and nitrite into less-harmful nitrate. The authors tested the convenience and efficiency of three methods for the initial inoculation of aerobic biofilters. Only the system stocked with fish at low density showed stable nitrification and low ammonia and nitrite concentrations. A system equipped with used filter media started nitrification immediately, while ammonia and nitrite concentrations in a system with a commercial solution containing nitrifying bacteria remained high.