Canister filters are as compact filters that sit outside of the aquarium and filter the tank water.

The water goes throught a variety of processes and usually begins with a mechanical prefilter that filters out detritus and other particles.

The water then enters a carbon filter which chemically removes some of the pollutants. The final stage is usually a resin or other filter that is just for polishing your water.

It is very important to maintain these filters regularly and clean them out. There are many people who hook these up and just forget these filters. They seem to think that the tank looks okay, so they just leave him alone. But in truth oh the water chemistry of the tank may be changing without you noticing it in the tank is slowly and in the process of rebalancing and then at some point at a tipping point it will rebalance in the favor of hair algae and you suddenly get an explosion hair algae and then you wonder what the heck happened !!

So if you get one, keep it clean. We don't think they are the best way to filter your water, but some people like the simple all-in-one approach. Just make sure you keep it clean.