Canister filter

A canister filter is an external filtration unit, usually kept in the cabinet below the aquarium, that pulls water through tubing, pushes it through several stages of filter media inside a sealed canister, and returns clean water back to the tank. Because the canister sits outside the aquarium, it can hold far more filter media than a comparable hang-on-back filter, which supports a larger colony of the beneficial bacteria that process ammonia and nitrite.

Most canisters use a layered media setup, typically a mechanical stage such as filter floss or sponge to catch debris, followed by biological media to house bacteria, and sometimes a chemical stage like activated carbon for specific problems. The sealed design also runs quietly compared to many hang-on-back filters, and the flow can be aimed through a spray bar or lily pipe to keep the water surface moving without disturbing a planted layout too much.

When choosing a canister filter, size it to the tank's volume and stocking level rather than buying the smallest model that technically fits the tank size listed, since planted tanks with heavy bioloads benefit from extra flow and media capacity. A common mistake is skipping regular maintenance because the canister is out of sight, which lets media clog with debris and gradually reduces flow and filtration quality over time.