Water is recycled and reused after mechanical and biological filtration and the removal of suspended debris and metabolites in a recirculatory aquaculture system (RAS). This technology is utilized for high-density fish rearing using the least amount of land and water possible. Unlike other aquaculture production systems, it is a high-density intensive fish culture. In this system, fish are often reared in indoor/outdoor tanks in a controlled environment, rather than the traditional method of growing fish outside in open ponds and raceways. By recirculating water back to fish culture tanks, recirculating systems filter and clean it. The approach is based on the combination of mechanical and biological filters, and it can be applied to any aquaculture species. Only enough water is added to the tanks to compensate for splash out, evaporation, and waste material flushing. Because wastes and uneaten feed are not simply thrown into the ambient environment like they are with other culture systems and foreign species, and infections are not introduced into the ecosystem, RAS has a lower environmental impact.