Abstract: An underwater filter for aquaria has a base portion (1) on which several similarly constructed filter cassettes (6) are located above each other. The filter cassettes (6) are made to be essentially rectangular of block-like shape and have two filter chambers (68, 69) as well as a clear-water (66). The clear-water chamber (66) is positioned between the two filter chambers (68 and 69) and is in flow communication with the base portion (1), the covers (61, 62) formed with apertures (610, 620) therethrough of the essentially rectangular block-like cassette (6) are removable, whereby access to the filter materials in the filter chambers (68 and 69) is possible. In operation, the aquarium water to be filtered is sucked by means of a pump connected to the base portion through the slits (610, 620) in the filter chambers (68 and 69) and from there into the clear-water chamber (66) and connecting thereto into the base portion (1) and then returned to the aquarium.