Aquarium Power filters, sometimes called Hang on Back Filters or HOB Filters, are most popular in freshwater aquariums, but are also widely used in small saltwater & marine aquariums as well. An aquarium power filter combines a circulating pump and mult-stage filtration into one self-contained unit that in most cases hangs outside the tank with a collecting tube attached to bring water into the filter. Power filters are particularly attractive because all components, including replacement filter cartridges and the pump itself can easily be cleaned or replaced with minimal disruption to the aquarium and its inhabitants. When ordering your power filter, we urge you to order replacement filter cartridges so that you can always keep your power filter operating at maximum efficiency.