• Choose the right aquarium filter

Undergravel Filter
The Undergravel Filter is one of the simplest types of aquarium filters. It consists of a slotted plastic plate that rests on the bottom of the aquarium and covers the entire bottom of the tank. There is a space underneath the plate that allows water to flow through. Suitable aquarium gravel is layered on top of the plate to a depth of two to four inches. There are one or more lift tubes that come out of the plate and extend up through the water to near the surface. An air pump or water pump such as a powerhead is used to pull water up through the lift tubes. The effect is to circulate water down through the gravel, under the filter plate and up and out the top of the lift tubes. The surface of the gravel and other surfaces of the filter plate are where the ammonia-eating bacteria will live. As long as the water continues to flow through the gravel and out through the lift tubes, the bacteria can survive and multiply. The more bacteria that can grow, the more ammonia they will consume. It's like a built in sewage treatment plant. But if the water stops flowing, the bacteria will die and the ammonia will accumulate.

Power Filter or Outside Power Filter
The Power Filter or Outside Power Filter is a very common type of filter for aquaria. It consists of a plastic box that hangs on one side of the tank on the outside. Water is pumped up through a lift tube, out of the tank and into the box. The water flows through various filter media and then is returned to the tank, usually by an outflow chute. The principle is essentially the same as the undergravel filter, except that the gravel is replaced by some other, artificial media that bacteria can grow on. This may be a plastic screen, a sponge, ceramic cylinders, or any number of other materials. Each brand has its own type of media. Again, the bacteria will live and consume ammonia as long as there is oxygenated water flowing over them.

Canister Filter
The canister filter is similar to the outside power filter except that it's usually placed on the floor or inside of a cabinet stand, rather than hanging on the side of the aquarium. It consists of a sealed container or canister with two plastic tubes that carry water from the aquarium to the canister and from the canister to the aquarium. Inside of the canister are the various types of filter media as in the power filter. And as in the power filter, the ammonia-eating bacteria will live and consume ammonia as long as there is oxygenated water flowing over them.

Sponge Filter
Sponge filters are made in various shapes and sizes for various applications. They consist of a large piece of foam material with a central core and lift tube that allows water to flow up and out of the sponge. The entire filter is placed inside of the aquarium just as in the undergravel filter. Frequently they have a weighted base to keep them resting on the aquarium floor. The filter can be driven by an air pump or by a powerhead if the lift tube is large enough. The foam or sponge material is the media where the bacteria can live. Water is drawn through the sponge and into the central core where it is pumped out to the aquarium again.

Other Filters
There is a wide variety of other types of aquarium filters. Every aquarist develops their own preference for certain types or combinations of filters. Any filter will work as long as it can support a large enough colony of ammonia-eating bacteria to consume the waste that your fish are producing. The more fish you have, and the more they eat, the more bacteria you must have to consume the waste. You can use more than one filter and more than one type of filter on your aquarium. Some aquarists like to have more than one filter as back up in case one filter fails.