Abstract: An inlet conduit in the form of a rigid, perforated support for gravel in the base of a fishtank is communicatively connected to a chamber having a first portion housing filter means and two second portions on opposite sides of the filter means. A tube formed with apertures throughout its entire length extends longitudinally across the lower end of a convergent nozzle which has an elongate cross-section and extends upwardly from the filter means. The tube is supplied with pressurized air through a hose and, as a result, air bubbles issue at a uniform rate across the whole width of the nozzle and, as these bubbles pass upwardly through the nozzle, induce flow of water through the gravel, through slots formed in the inlet conduit, and through filtering material enclosed in the filter means to thereby clean the water drawn through the nozzle.