A filtering device, in particular for fish basins, having a housing (10) encompassing a filter chamber (20) and a filter drum (30), which is rotatably disposed in the filter chamber (20) and is loadable with water to be purified, about which a filter belt (40) is guided along part of its circumference from a dispensing roller (42) to a receiving roller (41) that is driven by a paddle wheel (5) disposed outside the filter chamber (20). The paddles (51) of the filter wheel are activated by unpurified water in such a way that the filter belt (40) is wound from the dispensing roller (42) onto the receiving roller (41).