Aquarium canister filters are a great tool to help keep a fish tank clean and comfortable for its inhabitants. A proper filtration system is important for any aquarium to ensure that fish reside in a livable environment. Regardless of how often water is changed, an aquarium is a closed environment and contaminants can build quickly. In a natural setting, nitrates and ammonia released in a fish's excretion would be absorbed by plants and used as fertilizer. While live plants can help keep a fish tank clean, aquarium plants have shorter roots and the stagnant water of an aquarium does not allow for toxic chemicals to disperse in natural currents as they would in a lake, river or ocean. Additionally, the population of a fish tank is usually denser than in the wild, and ammonia levels can quickly climb to far greater heights.

Aquarium canister filters are a type of external filter. Unlike internal filters, they are attached to the outside of the tank, making them easier to clean and maintain. Many fish owners prefer canister filters to other kinds of external filters because they offer the choice of a wide variety of filler materials. Different materials are more effective at filtering different contaminants from the water and the filler can be chosen based on the type of fish living in the tank and the types of materials found naturally in the water. Fish tank canister filters can be quite large, providing a greater amount of filter material, and a greater efficiency, without taking up any space inside the aquarium. Because they are large and complex, however, they can be relatively expensive, compared to most internal filters and it can be difficult to clean the insides of the tubes that run between the aquarium and the filter.

Aquarium canister filters comes in a range of prices, the cheapest costing less than thirty dollars, and the most expensive costing over five hundred. One of the most affordable models on the market is the TOM Aquarium Rapids Mini-Canister C-80, which costs between twenty and thirty dollars, depending on where it is purchased. This filter is suitable for smaller tanks, ideally around ten gallons, although it can be used with tanks that hold as much as twenty gallons of water. The C-80 is small enough to hang on the back of the aquarium and provides a three-stage filtration system, using a dense floss roll to trap solid debris, an activated carbon filter pad to absorb dissolved organic contaminants, and porous ceramic cylinders to neutralize ammonia and nitrates in the water. This filter is very quiet and can be used with both freshwater and saltwater systems, and it is a good basic filter for a small tank with only a few fish.

The Cascade 1000 Canister Filter is a great mid-range filter for larger aquariums. Costing around one hundred dollars, it has 265 gph of flow and can service a tank of up to one hundred gallons. It features a push button self-primer and independent 360 degree rotating valve caps, for ease of use. Like the TOM C-80, the Cascade 1000 uses three stages of filtration to remove solids, organic and chemical contaminants from the water. Its large filter tray means that many filtration media can be used at a time, allowing them to be replaced less often. Additionally, unlike the C-80, this model features flow-rate control valves that make set-up and maintenance easy.

Those who want a top-of-the-line product may find what they are looking for in Fluval's G Series Canister Filters. These filters are all-inclusive and fully computerized. G Series filters analyze the water as it flows through the canister. They can pinpoint the organic and chemical content and will display a reminder when it is time to change the filtration media. They also utilize a double-walled canister design that makes them run completely silently. The downside, of course, is the cost, which ranges from four-hundred to six-hundred dollars, depending on the size needed. For those who can afford them, however, the Fluval G Series filters are the best in aquarium canister filters.