Learn About Aquarium Canister Filters
Water enters and passes through a filter pad that traps large particles and debris (mechanical filtration). It is then routed through several media trays that hold water-cleaning media: First a layer of carbon that takes out colors, odors, chlorine and early-stage ammonia in the water by actually absorbing it into the carbon. (chemical filtration) Then the canister works its magic. The "bacteria bed" of most aquarium canister filters is in a chamber holding bio-media, ceramic rings, bio-stars, bio-balls or some other type of media made to house bacteria cultures that will change the toxic nitrites into less-toxic nitrates (biological filtration).
Depending on the size of the aquarium, the canister may hold two, three, even four trays of media.
A good aquarium canister filter will be easy to shut down and easy to start again without a lot of priming or water spills, so that when it is time to clean or adjust media you are not faced with a sloppy or frustrating job.
A good aquarium canister filter should have flexible media trays so that you can choose what media you want to use in each tray. That will allow you to use charcoal, zeolite, toxin-absorbing pads, water polishing pads, and/or bio balls and rings in any quantities and any time your tank may need it. Some even offer water polishing micron pads that will make your water virtually sparkle as well as filter out many pathogens such as ich tomites (that cause ich disease) and algae cells.
A good aquarium canister filter should be quiet when operating and require minimal maintenance. Most need to be cleaned or adjusted only 2-3 times per year. When it does require cleaning or media adjustment, it should be easy to work with. Baskets that hold media should be easy to get in and out. The mechanical-cleaning pads and chemical-filtering carbon should be very easy to get to, as they are the most frequently cleaned or changed. (Many aquarists use an ancillary tank-mounted power filter for added mechanical and chemical filtration because you can change those functional cartridges with just a quick lift-out and replacement of the cartridge right from the tank top without disturbing the canister filter at all.)
A good aquarium canister filter should have a high-quality impeller. Most name brands today have impeller motors that will last for 10 years or more.
Larger tanks and more heavily populated tank needs much more biological filtration than smaller or sparsely populated tanks because more fish produce more waste. An aquarium canister filter is overkill for a 10 gallon tank. However for 20 gallons and up, it is a huge benefit: better filtration, fewer fish deaths, and MUCH less maintenance.