A compact aquarium filter 10 of the type including a roll of filtration material 40 which automatically advances as it becomes dirty. The filter includes a bypass outlet (50, fig 2) which can be opened to allow water through the filter body without being passed through the filtration material. The filtration material passes around a removable web guide 28, the in-use part of the filtration material changing direction around the web guide to fold back on itself. The filter inlet 20 is in the base of the filter housing 12. A motorised take-up spool is also disclosed, in which a spool assembly substantially surrounds a motor assembly, to provide a highly compact unit.
