An aquarium canister filter is a type of external filtration system for aquariums. It's typically a sealed canister with several compartments or trays inside, filled with various types of filtration media, such as mechanical, chemical, and biological media. The water is pumped into the canister, flows through the media, and is then returned to the aquarium.

The main advantage of a canister filter is that it allows for a large amount of filtration media to be used, which results in more thorough filtration of the water in the aquarium. Canister filters are also very versatile and can be customized with different types of media to suit the needs of the specific aquarium.

Canister filters are typically designed to be placed under the aquarium, drawing in water from the aquarium and returning it back after filtering. They are also suitable for larger aquariums due to the high flow rates and high head pressure of the pumps that power them.

In terms of the process, a typical canister filter will first go through mechanical filtration, where larger debris and particles are trapped by a filter pad, then the water will flow through biological filtration media, where beneficial bacteria will grow, breaking down harmful ammonia and nitrite to less toxic nitrate, and lastly chemical filtration, where substances such as activated carbon can be used to remove dissolved impurities and discoloration in the water.