One of the most basic and easy to use aquarium filters is the power filter, or hang on back filter. Most aquarium power filters have a slot for a filter cartridge that is usually filled with activated carbon and warped in aquarium filter pads. Every 4 to 6 weeks, you just pull out the filter cartridge and add a clean one. Before you replace the cartridge, you may want to quickly rinse the new filter cartridge to remove any carbon ash. Many aquarists also like to add a squirt of beneficial bacteria, to replenish the bacteria that were living on the old filter cartridge. Some manufacturers make bulk filter cartridge packs that allow you to assemble your own cartridges at a discount.