NOVELTY - At least partial reduction of phosphate products in a closed estuarine or marine recirculating aquaculture system (RAS) comprising brackish or salt water, comprises denitrifying the brackish or salt water of aquaculture system, in the presence of a media substrate, under conditions to form a denitrifying biofilm on the media substrate and to form phosphate salts on or in the denitrifying biofilm; and removing the media substrate bearing the biofilm and phosphate salts from the aquaculture system.    USE - The method is for at least partially reducing phosphate products in a closed estuarine or marine recirculating aquaculture system (RAS) comprising brackish or salt water. It is useful for anaerobic treatment of waste material of aquaculture species in closed estuarine or marine RAS, useful for producing methane gas. The aquaculture system is used for farming fish selected from gilthead sea bream, cobia, or sea bass. (all claimed)    ADVANTAGE - Both the substrate and phosphates can be reused or recycled. The method, which is efficient and cost effective, is an alternative to exclusive use of expensive chemical approach or less efficient biological approach. Nitrogenous wastes in estuarine or marine RAS are eliminated through the action of nitrifying and denitrifying biofilter units that rely on oxygen and reduced organic compounds, respectively, for their activity. The recovered soluble phosphate salts can be recycled as a value added product in prepared artificial seawater, for production of fish food or as fertilizer. The phosphate can be recycled in prepared artificial seawater, for production of fish food or as fertilizer. Recovery and recycling of the phosphate minimizes the environmental impact of RAS by preventing the release of phosphates into a waste stream and the local environments. The method minimizes or prevents phosphates from accumulating in a closed recirculating system to toxic levels.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for:    (1) closed estuarine or marine RAS comprising brackish or salt water comprising nitrate products and phosphate products; a rearing tank comprising an aquatic species; and a denitrification unit comprising a media substrate, where denitrification unit denitrifies the brackish salt water, forms a denitrifying biofilm on the media substrate, and forms phosphate salts on or in the denitrifying biofilm; and    (2) anaerobic treatment of waste material of aquaculture species in a closed estuarine or marine RAS comprising brackish or salt water, comprising denitrifying the brackish or salt water of aquaculture system, in the presence of media substrate, under conditions to form a denitrifying biofilm on the media substrate and to form phosphate salts on or in the denitrifying biofilm to sequester phosphate; removing the media substrate from the aquaculture system; and removing waste material from the aquaculture system and anaerobically digesting the same to produce methane gas.