A canister filter is a piece of equipment that mounts to the exterior of an aquarium with one piece inside. There are adjustable inlet and outlet fittings that allow the water to flow in and out of the filter. There is also an in-tank pump to push the water into the canister for filtration. Once the filter is full, the pump forces the water out again. But before this it will pass through a range of filtration layers. Membranes inside work to capture bacteria and debris, only letting clean water back in again. The filter needs plenty of maintenance, and you should clean all the individual components.\r