Three kinds of filtration are needed to maintain a healthy aquarium: mechanial, biological, and chemical. All the variety of filters on the market basically offer offer one or more of the three basic kinds of filtration needed to maintain good water quality. Mechanical filtration is the removal of particles from the water. Biological filtration is the conversion of fish waste products from toxic compounds to non-toxic ones. That is, the initial compound, ammonia, which is very toxic to fish in even very small amounts, is converted to nitrite, another toxic compound, and then to nitrate, which is comparatively less harmless to fish (refer to cycling for more information on biological filtration). Chemical filtration is the removal of some types of dissolved organic compounds in the water. However, not all such compounds are removed by chemical filtration, and for this reason, among others, regular partial water changes are an important part of keeping fish.

Filters on the Market

Undergravel Filters (UGF) - The undergravel filter, once a mainstay of aquaria hobbyists, today is arguably one of the most controversial topics in our hobby. UGF's consists of slotted plating that covers the bottom of the tank. These plates are raised slightly providing a small clearance between the plating and the tank bottom. On top of the plates sits a 1-2 inch layer of gravel (substrate) which serves as the filter media. The gravel should be coarse enough to not clog the filter plate slots, but not so coarse that it is no longer a filter media. Rising from the UGF plates are lift tubes. UGF's are powered by air pumps or powerheads (preferably powerheads as air pumps do not generate the needed water flow). Water is drawn through the gravel media, down through the plating, up through the lift tubes, and back into the tank. The UGF acts as both a biological and a mechanical filtration system using the gravel (mechanical) and the bacteria colony on the gravel media (biological). With advent of sophisticated power filters and canister filters, many believe that UGF's are obsolete and in fact, a ticking time-bomb. This is because without proper knowledge, handling, and maintenance... dead spots form under the plates and gravel and bad bacteria begins to grow. Personally, I feel that the risks far outweigh the rewards for an UGF. It can be a fabulous and cheap filtration system, in the hands of a highly experienced hobbyist who know how to have success with it through decades of use. But in the hands of one less experienced or less diligent in routine maintenance, it is a disaster waiting to happen. A powerhead should be used to power a UGF to ensure adequate current. And the gravel should be vacuumed well during routine water changes. If a UGF is to be used... it is best to use a UGF in conjunction with another filter, a power filter or cannister filter. However, I would recommend just scrapping the UGF to begin with and going with a power filter or cannister filter (why play russian roulette).

Pros: inexpensive, great biological filtration capacity
Cons: labor-intensive to maintain, potential to go bad and build up "dead" areas and toxic environment,not suitable for most planted tanks

Corner Filters/Box Filters - A corner/box filter consists of a plastic box/container that is filled with filter media: filter floss and carbon. They are designed for mechanical and chemical filtration. The entire unit is submerged in the tank and powered by an air pump. Water is drawn through the box and the filter media and expelled back into the tank. It has a poor capacity for filtration, and nowadays typically only would be considered for fry tanks or hospital tanks (using media from an established tank for a quick emergency tank setup). It would be a decent choice for a fry tank due to its gentler flow rate. Even then, I would suggest using a sponge filter for hospital tanks and fry tanks. Keep a sponge filter in your main tank and you'll have a seeded sponge filter ready for emergency setups.

Pros: inexpensive
Cons: frequent replacement of media, weak filtering capacity

Sponge Filters - A sponge filter is a very simple, yet effective filter. It consists of a sponge with tubing and powered by an air pump (or powerhead). Water is drawn through the sponge (with the bacteria colony for biological filtration), up the tubing and expelled back into the tank. Like the corner filter, it is also entirely submerged. The sponge filter, however, has a much greater biological filtering capacity. They are easy to setup and easy to maintain. To clean the sponge filter, merely rinse the sponge out in old tank water (to preserve the bacteria colony). Sponge filter are well suited for use in a hospital tank or fry tank, as the water flow is not overpowering for fry or weak/sick fish. As mentioned above, a ready seeded sponge filter is great for emergency setups of hospital tanks.

Pros: inexpensive, easy to set up/maintain, high biological filtration capacity
Cons: little mechanical filtration, inadequate for larger tanks

Trickle (Wet-Dry) Filters - A trickle filter sits underneath the aquarium stand. Water is gravity fed to the trickle filter where a bacteria bed is supported. This bacteria can consist of bioballs, a biowheel, or other biological media. The key is that all of the media should be held out of the water. Water is allowed to "trickle" over the biomedia while being exposed to air. The result is an extremely large amount of bactria growth, capable of supporting extremely large fish loads. Most trickle filters will have a prefilter which serves for mechanical filtration. If you decide to design your own model, it is very important that all of the biomedia be out of the water. Submersed biomedia defeats the purpose of a trickle filter and you may as well use a sponge filter.

Power Filters – Power filters are external filters, specifically, they hang on the back of your tank (sometimes called hang-on filters). Water is drawn up by a motor, through an intake tube and into the power filter; consisting of a plastic box with compartments for filter media (sponges, carbon, charcoal...). It allows for mechanical, biological, and chemical filtration. The water exits the filter compartment via an overlow opening, resembling a waterfall. They are easy to maintain and quiet to operate. The media can be interchanged, replaced, or rinsed out, as appropriate. In my opinion, these are the best filters for small to medium sized aquariums due to there capacity for filtration, ease of use/maintenance, and moderate price. It is often best to double up on the sponge media to increase its biological filtration capacity.

Pros: moderately priced, easy to set up/maintain, choice of filtering media
Cons: inadequate for larger tanks

Bio Wheel Filters - Biowheels resemble the "paddle-wheels" on a paddle boat. They consist of pleated filter material on a cylindrical wheel. The wheel is powered from water expelled from either power filters, spray bars from cannister filters, or powerheads. As the wheel spins, it exposes the bacteria colony to the air allowing for higher levels of oxygen exchange and thus a larger bacteria colony is able to grow. Biowheels offer a tremendous amount of biological filtration and is relatively maintenance free. The filter pad can be rinsed out periodically in old tank water and changed as appropriate. Some power filters come with a biowheel attachment, thereby enabling mechanical and biological filtration.

Pros: inexpensive, relatively maintenance free, large biological filtration capacity
Cons: not a stand-alone filter (must be used in conjunction with other filers)

Cannister Filters – Cannister filters are rather pricey, but offer high capacity filtration of all three forms: mechanical, biological, and chemical. They are efficient and quiet. Water is drawn out of the tank via an intake tube, through piping/hosing, and into an external cannister (typically located below the tank). Within the cannister are several compartments for all types of filter media imaginable. The filtered water is then sent out through more piping/hosing and expelled back into the tank via a spray bar. Cannister filters provides powerful filtering, at a high capacity, and are extremely efficient filters. The only possible drawback is that some cannister filters can be a chore to disconnect to maintain and restarting the unit can be trying at times.

Pros: efficient, powerful, choice of filtration medias to suit one's needs, quiet, handles large bio-load
Cons: expensive, chore to maintain/restart

Fluidized Beds - These filters are growing in popularity and deservingly so. They consist of a cylindrical pipe filled with sand. Water is pumped through the filter and the sand, resulting in massive bacteria growth. They are an amazing source of biological filtration contained in a very small unit. Aquariums up to several thousand liters can be filtered with these units. They do require a prefilter to remove particulate matter. The prefilter should be cleaned weekly to allow for the filter to maintain its water flow rate. These units are best used in conjunction with a power or mechanical filter of a high flow rate.



Pros: HUGE biological filtration capacity, quiet, handles large bio-load
Cons: expensive, needs a powerhead/or cannister filter as a mechanical pre-filter

Powerheads - A powerhead is fundamentally not a filter; however, powerheads can be fitted with filter attachments to the intake valve. With an attachment like this (such as a quickfilter by Aquaclear), the powerhead not only serves its purposes for water circulation/aeration, but it doubles a a filter also. Also, powerheads are often used in conjuction with UGF's (undergravel filters) for more powerful current flowing through the plates.