A canister filter is a self contained remote unit that connects to the aquarium by hoses. The water is moved through the filter media in the body of the sealed unit and then returned to the tank. One of the advantages of this design is that they can be used in installations with very limited space. They have a much larger capacity for filter media than the HOB units but the media is never directly exposed to the air, thus limiting the efficiency of the bio media inside them. They have a lot higher flow rate than the HOB units and can go for much longer periods of time between servicing. Canisters are a sealed system and this allows them to provide a greater mechanical filtration than other common types of filters although this is limited by their lack of a true pressure rated pump. They are unfortunately much harder to clean when the time comes for a service. The units have to be turned off…disconnected from the hoses…and moved to another area to be cleaned. The difficulty of this process varies from brand to brand but in general they are all fairly close to the same. After servicing is complete the canisters must be purged of air to operate properly and with some units this can be a problem.