Canister Filters are a large powerful aquarium filter. They are normally cylinders that stand up with locking lids with a series of filter chambers which water is pushed through. Most units have about three different chambers, chemical, biological and mechanical depending on the filter media used. However all generally perform the same functions. Most models have internal pumps the force water through chambers and returned into tank. Ideally Canister Filters are a good source of controlled filtration for salt and freshwater aquariums, With a saltwater aquarium a canister filter is generally good on smaller size aquarium because you are limited to handle the large bio-load