All aquariums require biological and mechanical filtration to maintain a healthy environment. The aquarium filter also assists in aerating the water. To effectively maintain an aquarium, a filter should run all the water in the tank through the filter a minimum of four times each hour. Small to medium aquariums do well with power filters or a UGF, while canister filters are better for larger aquariums.
Box (Corner) Filters
Also called corner filters, these were the first aquarium filters available for home aquaria. Although no longer used as often as in the past, they are very inexpensive and can be loaded with a variety of filter media. Corner filters are often used for hospital tanks because fish owners don't wish to invest a lot of money setting up a tank that is used infrequently. Their less powerful intake flow makes them popular for use in breeding tanks with tiny fry.
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Canisters are powerful aquarium filters best suited for medium to large tanks. Because they are outside the tank, they may be easily concealed behind on beneath the aquarium stand. Canister filters force water through the filter media, rather than allowing it to flow past it as other filters do. This makes them ideal for heavy loads. Adding a biowheel increases the biological filtration capacity of the aquarium filter.
On the negative side, canister filters are difficult to take apart for cleaning and maintenance, and difficult to get primed and restarted afterward.
Diatomic are specialized aquarium filters that 'polish' the water by removing very small particles. They are most often used when fine particulate matter, such as diatomic algae, is a problem.
Because a diatomic filter is used only for specialty situations, some standard filters are made with diatomic inserts so they may serve a dual function when needed.
Fluidized Bed Filters
Relatively new, these aquarium filters are very efficient biological filters utilizing sand as the filter medium. The small particles provide a high surface area for the bacterial colonies. Although it takes a bit longer to mature initially, the fluidized bed is an excellent biological filter that can be used in any sized aquarium.
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Without a doubt, power filters are the most commonly used aquarium filters. The standard power filter hangs off the back of the aquarium, is simple to install, easy to maintain, and provides both mechanical and chemical filtration. Power filters may be combined with a biowheel to provide increased biological filtration.
The sponge filter is fitted over a tube from a power head or air pump. As water is forced through it, bacteria will grow and establish a biological filtration.
Sponge filters also provide mechanical filtration, although they clog quickly if there is excess debris. They are excellent for tanks with fry, as the sponge prevents young fry from being sucked through the pump. Sponges are good in a hospital tank, as a sponge from an established aquarium quickly provides the tank with nitrifying bacteria.
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Trickle (Wet/Dry) Filters
Also called a wet/dry, trickle filters are intended to expose the water to as much air as possible. This is accomplished by allowing the aquarium water to trickle over a container of media such as plastic balls, strands, or floss. Although originally popular among saltwater hobbyists, trickle filters are becoming increasing popular in freshwater aquariums. The biggest drawback is the fact that they clog fairly easily. The use of a mechanical pre-filter eliminates or reduces that problem.
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UGF (Under Gravel Filter)
The UGF (Under Gravel Filter) is another aquarium filter that has been around for a long time. It utilizes a plate filter that is placed under the substrate, then an air pump is used to pull the aquarium water down through the substrate, taking the particulate matter with it. The UGF is inexpensive, easy to set up, and once running, is relatively maintenance free. On the downside, UGFs tend to clog, and are not a good choice for an aquarium with live plants.
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