Mechanical fish tank filtration traps solid refuse from the tank, however, they are still part of the system until it is replaced or cleaned. The most common media for this is plastic foam, filter floss, and gravel or sand.

Chemical filtration uses media that alters the chemical makeup of the water. The alteration of pH using lime rich material or peat can reduce hardness. Chemical media can also trap solids by binding with harmful chemicals causing them to be neutralized.

Biological filtration is where natural populations of the bacteria that operate the nitrogen cycle are enhanced by providing the conditions they require. This includes surfaces to colonize, supplies of wastes to process, and a constant supply of oxygen. This filter type provides living space for the fish and the flow of water provides a consistent supply of oxygen and wastes. This type of filtration occurs in any chemical or mechanical filter that has been left be for a period of time to create a bacterial population. The media used also have mechanical effects.

The nitrogen cycle should be carefully considered when you are planning to maintain a filtration system. It takes about two weeks for the nitrogen cycle to get comp lately functional in the new system. Fish cannot be put into the tank until after such a period of time because of nitrate and ammonia toxicity. If the current nitrogen cycle is interrupted there will be a risk of toxicity again. This usually happens when dirty filters are cleaned. In fact, most of the dirt on an old filter is the inconsequential residue left by processing bacteria. It doesn’t do anything to improve the quality of the water when you remove it, but by eliminating the bacterial population you could cause chaos. Regardless of this potential chaos it is necessary to clean filters before they get clogged.

Canister filters are self-contained units that have a container or filter chamber for media and an electric pump to circulate the water. They are usually external with inlet and outlet pipes to and from the tank. They can also be internal where they have slits to allow faster into the filter and an outlet from the pump. They combine mechanical and biological filtration, and chemical filtration is optional. Special external filters use diatomaceous earth to optimize water clarity by removing particles mechanically but these are expensive and need often maintenance because they clog quickly. Canister filters are biological and mechanical, as well as sometimes chemical. The water is siphoned out of the tank into the canister filter, and then pumped through the filter media back into the tank via a spray bar.

Undergravel or UG filters are made of a plastic plate situated between the tank bottom and the substrate with one or more uplift pipes. Water is pulled down through the substrate and returned via the uplift. UG filters are often powered by the airlift principle, power heads, or an external canister filter. The substrate should be about five cm deep for effectiveness, and acts as the filter medium. Its action is biological and mechanical as well as sometimes chemical. UG filtration advantages include that the inlet is the entire tank bottom so wastes cannot escape processing.

Trickle filters are made up of trays with the bottoms perforated. These are partially filled with filter media and stacked above the aquarium. It is used in conjunction with an external filter canister. The return is sprayed onto the top tray and trickles back into the aquarium via the perforations. This exposes the water to air which increases oxygen uptake and bacterial activity. Another advantage is that it has the capacity to be increased by means of multiple trays without needing to increase the water flow rate. Trickle filters can be positioned underneath the aquarium with water siphoned into the filter and pumped back. These are usually sited on top of the tank and fed by the return for a canister filter.

Box filters can be internal or external and work by driving the airlift principle. These have pretty much been taken over by more complex systems, but the internal type can still be used for small aquariums and tanks with minimal loading.