Filtration

This is the most basic type of filter which consists simply of a sponge through which water is drawn by rising air bubbles from an air pump. The sponge acts mechanically, and once matured, biologically. These filters are ideal for small breeding and rearing tanks where a power filter might suck fry into the filter inlet.

These are also air powered and come in a number of sizes to suit small to medium sized tanks. The airflow draws water through the chamber of the filter which can be filled with the desired media. This type of filter can be moved between tanks to provide additional mechanical filtration using filter wool for example, or for adsorption of specific waste products using carbon or specialised adsorbent resins.

This type of filter utilises an electrically powered centrifugal pump to draw water through the filter material, normally sponge. They may have additional chambers for adsorptive media or additional mechanical/biological media. They generally create a more powerful circulation than air-powered filters. They are ideal for aquariums in the 2ft - 4ft size range, and are useful in combination with external canister filters for large aquariums. They also provide useful extra mechanical filtration when used in conjunction with undergravel filtration.

Hang-On-Tank or Hang-On-Back filters are popular in the US and other regions, and are similar to internal power filters in terms of design principle and the size of aquarium that they are suitable for. Some use disposable cartridge systems to facilitate easier maintenance. The cartridge system may not cater for bio-filtration adequately, so models which include a sponge filter or an add-on like the BIO-Wheel are preferable.
The BIO-Wheel is an efficient bio-filter which can be added to different filtration systems and functions like a wet-dry filter (see below).

These work on the same principle as internal power filters, but are positioned outside the tank and draw water into the filter through an inlet tube and return the water via another tube, which can be fitted with a spray bar to increase aeration. They avoid the need for an unsightly filter inside the tank, but this has the disadvantage that they require priming before use, although modern designs now incorporate self-priming devices. The media capacity is much larger than internal filters and they are therefore suitable for much larger tanks. They normally incorporate areas for several media types so the media can be tailored to suit the requirements of individual tanks. They are a good choice for larger planted tanks.

Undergravel filters work by drawing water from the tank down through the gravel substrate, which acts as a mechanical and biological filtration media. This gives a huge surface area for filtration. The gravel does need to be 'vacuumed' occasionally however, to prevent the build-up of excess dirt in the gravel and beneath the filter plates.

The water is drawn through the undergravel filter plates and into uplift tubes by an airpump or powerhead. Powerheads will give a more rapid turnover and stronger circulation. Reverse-flow undergravel filtration (RUGF) works by drawing water from the aquarium, through a prefilter of some sort (which could be anything from a simple sponge to an external canister filter), and forcing it DOWN the uplift tubes and up through the gravel. This avoids the problem of excessive debris building up in the gravel and beneath the filter plates.

Sump filters are large (often home-made) outside filters placed below the main tank. They may consist of a smaller aquarium tank or other container filled with filter media. Heaters may also be placed in this sump tank to keep them out of the main tank. They may be quite complex in design and are especially useful for marine tanks because additional equipment, such as protein skimmers, can be placed in them. They also have the advantage of increasing the overall volume of the system.

These may also be home-made, but are also available commercially. They are very efficient at biological filtration and work on a principle similar to many sewage treatment plants. Water is sprayed over the surface of the media, often in several layers, so that a 'pulse' of highly oxygenated water is delivered to the media. Other wet-dry systems are incorporated into canister filter type designs.

These are specialised biological filters which rely on a constantly moving bed of sand as the filter medium. This provides a huge surface area for bacterial colonisation and the constant rubbing together of sand grains ensures that loosely attached debris and dying bacterial colonies are removed. They act only as biological filters and require a mechanical prefilter, such as a powerhead cartridge, or an internal or external powerfilter.

These are often used for short term mechanical filtration to improve water quality. The media used clogs quickly, trapping smaller and smaller particles. They need to be cleaned after each use but are very useful for giving water a quick polish.