NOVELTY - A recirculating aquaculture system comprises an aerobic biofilter reactor in fluid communication with an aquatic species rearing tank; an anaerobic biofilter reactor in fluid communication with and positioned downstream from the aerobic biofilter reactor; and mechanism for circulating effluent between the rearing tank and biofilter reactors. The media in the biofilter reactors are kept in motion within the water effluent.    USE - The aquaculture system is useful for producing aquatic species.    ADVANTAGE - The invention combines a separate anaerobic denitrification unit connected to a nitrification unit to provide a two-stage nitrogen removal system. The two-stage system reduces the water exchange rates and consumption of salt when maintaining a saline effluent.    DETAILED DESCRIPTION - A recirculating aquaculture system comprises an aquatic species rearing tank (12) for holding fresh water or salt water effluent; an aerobic biofilter reactor (14) in fluid communication with the rearing tank and comprising a tank having an inlet and an outlet for movement of the effluent, media to support the growth of aerobic microorganisms active in nitrification suspended within the fresh water or salt water effluent, and aerator to inject air or oxygen into the fresh water or salt water effluent for effective aerobic nitrification by the microorganisms; an anaerobic biofilter reactor (16) in fluid communication with the aerobic biofilter reactor and positioned downstream from the aerobic biofilter reactor, wherein the anaerobic biofilter reactor comprises a tank having an inlet and outlet for movement of the effluent from the aerobic biofilter reactor, media to support the growth of anaerobic microorganisms active in denitrification suspended within the fresh water or salt water effluent and mechanism for introducing a carbon source and optionally a nitrite source into the anaerobic biofilter reactor for effective denitrification by the microorganisms; and mechanism for circulating effluent between the rearing tank and aerobic and anaerobic biofilter reactors, wherein the media in the aerobic and anaerobic biofilter reactors are kept in motion with the fresh water or salt water effluent.    An INDEPENDENT CLAIM is also included for a method for removing nitrogenous products from a recirculating aquaculture system comprising housing an aquatic species in a rearing tank containing fresh water or salt water effluent contaminated with ammonia; introducing and circulating the fresh water or salt water effluent into an aerobic biofilter reactor and treating the effluent with aerobic microorganisms active in nitrification to reduce ammonia concentration and increase concentration of nitrates; removing the nitrified fresh water or salt water effluent from the aerobic biofilter reactor and introducing into an anaerobic biofilter reactor for treatment with anaerobic microorganisms active in denitrification to reduce concentration of nitrates; and introducing the denitrified effluent into the rearing tank.    DESCRIPTION OF DRAWING(S) - The figure shows an aquaculture system of the invention.    Aquatic species rearing tank (12)    Nutrient supply mechanism (13)    Aerobic biofilter reactor (14)    Valved conduit (15)    Anaerobic biofilter reactor (16)    Biomass carrier structures (17)    Filtration unit (18)    Discharge conduit (19)    Sump (22)