Guide to Aquarium Filters

One of the most important parts of an aquarium is the filter. While the environment of fish in the wild is refreshed by continual water flows, the entire world for your pet fish is held inside the glass or acrylic walls of the tank. The only fresh water source is you, when you change part of the old water for new.

Changing the water daily would keep the tank sparkling clean, but such activity puts a huge amount of stress on fish, making them more susceptible to disease. Proper filtration makes it possible to change water less frequently and makes the environment inside your tank healthy and comfortable. Without filtration, aquarium water rapidly becomes toxic, stressing and ultimately killing your fish.

Filters perform three functions. First is mechanical filtration, the removal of visible waste such as leftover food, excrement, etc. Second is chemical filtration, removal of additional toxins and chemicals such as ammonia by absorbing them in carbon or resins. Third is biological filtration, providing a breeding ground for the beneficial bacteria needed to eliminate the invisible chemical waste that makes the water dangerous for your fish.

Fish wastes raise the ammonia levels in aquarium water. A good culture of bacteria growing in, and being oxygenated by, the filter breaks down the ammonia into nitrites, which other bacteria then break down into nitrates. Excess nitrates are removed when you perform the recommended 25% water change every two weeks.

There are five primary filter types available for aquarium use. Each is appropriate for different situations and aquarium setups.

Sponge filter. This type can be used as either the primary filter, or as a pre-filter for another filtration system. Air tubing is connected to the sponge on one end and to an air pump sitting outside the aquarium on the other end. As a primary filter, it provides an excellent home for bacteria and does a reasonable job as a mechanical filter as well. The advantage of this type of filter is that it can be safely used in breeding tanks because it won’t suck up tiny newly-hatched fish. It’s also very easy to clean. On its own, this filter is best in smaller tanks or tanks that are not filled with too many fish, such as quarantine tanks. Use it as a pre-filter to keep bacteria from being destroyed when you have to change the aquarium’s main filter media.

Undergravel Filter. This type usually comes with the aquarium kits available at pet stores. It is made up of slotted plates that cover the floor of the tank, and has uplift tubes built in to each plate. Air tubing and air stones fit into these tubes and connect to an external air pump. At the top of the tubes are filter media inserts, usually carbon or resin, to provide some chemical filtration, though these must be changed frequently. The air pump creates circulation that pulls water down through the gravel that rests on top of the filtration plates. The circulation promotes bacterial growth in the gravel itself and the filter inserts clean up what the bacteria can’t. Unfortunately, if the gravel is too fine or too much overfeeding occurs, the filter becomes clogged and unusable until it is completely cleaned out. Even in optimal conditions, it is still important to vacuum gravel weekly to clean out the excess waste—this filter type does not do true mechanical filtration. This filter is best only in smaller tanks without a full load of fish, and using another filter type for mechanical filtration is a very good idea.

Internal Power Filter. This filter type can be used alone but is often used as secondary mechanical filtration for either an undergravel filtration system or a canister filter in a very large aquarium. It contains filter media—generally a sponge—and a pump, and fits inside the aquarium. It is an effective mechanical filter and provides some bacterial filtration as well.

Hang on Tank or Hang on Back Filter. These filters are growing in popularity as replacements for undergravel filters. This type consists of an intake tube, a pump, filter media (usually floss pads filled with carbon), and a slot where water is returned to the aquarium. Some of these filters include a porous, pleated wheel that provides another home for bacteria in addition to the filter media, so when the media is replaced, the bacteria don’t all disappear. The filter media provides mechanical, and some chemical, filtration. The pads used are very easy to change when they become full of waste. This filter is an excellent choice for aquariums smaller than 4' (1.2m) in length, or approximately 50 gallons (190L).

Canister Filter. This filter is the gold standard for large aquariums, because it can move a huge amount of water. It also provides room for multiple filter media types to be used at a time, so the combination of mechanical, chemical and biological filtrations are highly effective. Cleaning and changing filter media is not as easy with a canister as with other filters, but the excellent job this type of filter does on a big aquarium is worth the extra work to maintain it properly.

The key to proper filtration is to match the filter’s capabilities with your aquarium size. A filter that is too small will be a constant source of frustration as you change media too frequently and still have problems with water quality. Buy too much filter for your aquarium and you’ll not only spend more than necessary, you may end up with too high a water flow that stresses your fish. Choose the right filter, and your fish will be happier and healthier.