Abstract: A filtering system for use with fish-containing ponds and bodies of water is disclosed. More particularly, the filtering system includes chambers for harboring micro-organisms which clean organic waste from the water flowing therethrough from the pond. The chambers include by-pass conduits so that a given chamber may be taken out of service for cleaning without interrupting the flow of water through the rest of the system. Further, movable walls are provided so that several chambers contained in a single structure may be varied in number and in size according to a pond's requirements. Also included is an air manifold for use in supplying air to the micro-organisms.