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See also: Filtration FAQ

A filter is quite possibly the most important piece of equipment in any aquarium. It provides constant water circulation throughout the tank, removes debris, and providse a home for the majority of the nitrifying bacteria that neutralize ammonia and nitrite into less harmful nitrate.

The three stages of filtration[edit]

The material held within the filter (called the media) is split into three groups:


This is the first, simplest, and most basic stage of filtration. The goal is simply to trap particles (excess food, droppings, material scrubbed loose from the glass) in such a way that they can be easily disposed of from time to time. This stage of filtration usually consists of coarse to fine grained polyester floss.


The biological stage is where beneficial bacteria grow, and break down ammonia and nitrites into less harmful nitrates.

Any medium through which aquarium water is passed will eventually become a biological filter, but the goal in filter design is to create a stage which needs minimum cleaning (which will remove or kill the bacteria), passing on clean, clear water.

Common biological media include synthetic open-cell sponge, ceramic discs, or plastic materials formed to have a large surface area.

To periodically clean sponge-type media, simply take a bucket of aquarium water (from a partially completed water change), place it in it and squeeze it repeatedly until it appears relatively clear of mulm. Quickly return it to the water flow, to keep the remaining bacteria alive.


Chemical filtration consists of materials which achieve various goals in the water flow, such as absorbing organic chemicals that exhibit as odors or undesired colours in the water, reducing ammonia in new tanks, or even adding desired chemicals such as tannins.

They will often have a short lifespan and may not be intended to remain in the tank constantly.

Typical examples include:

  • Activated carbon - a granulised material, sometimes sold as a black sponge. This is intended to remove organic compounds as mentioned above.
  • Nitrate removers - a sponge or granule material. JBL ClearMec for example.
  • Additives which alter water chemistry - calcium or peat are common, to increase or decrease hardness, respectively.
  • Ammonia removers - these either absorb ammonia into the material or convert it to non-toxic form. JBL AmmoEX or Zeolite stones for example.
  • Silicate removers
  • Phosphate removers
  • Algae inhibitors

How do filters work?[edit]

Air driven[edit]

The simplest filters are driven by a small, steady supply of air bubbles provided by an external air pump via airlines and valves to control flow.

These consist of the basic plastic box in the tank with some floss and carbon sold to beginners, and the indispensable "foam" filters used by many aquarists for their biological filtration, and breeders due to that and the immunity to sucking fry or eggs into the filter.

A flow of air bubbles is introduced to the bottom of a plastic tube, and as they rise, water is drawn upwards, creating a suction at the lower end of the filter.

These filters are usually housed inside the aquarium.

Power filters[edit]

Power filters, which are usually housed outside the tank itself, use coils driven by mains current to spin magnets inside their chambers to drive water flow. They usually move far more water, and provide many more options for filter media.

Hang on back (HOB)[edit]

Hang on back, or "HOB" filters are rugged and simple and easy to clean, however, their return water delivery cannot be controlled in terms of direction and level. Most simply pour their water flow from "back to front" across the top of the tank.


Canister filters, while being harder to clean, provide the advantages of allowing tanks to be closer to the wall, and selecting water inlet and return location and direction. They are more arduous to service, although the better companies supply various valving accessories to make the task easier.

Wet/dry trickle filters[edit]

There are different variations of wet/dry trickle filters, but they all work on the same principle. The dirty aquarium water is first run through a relatively fine, disposable filter media to remove particulates. Then the water is allowed to drip through a very coarse media, such as pre-formed plastic balls or ceramic rings, while exposed to air. This allows the beneficial aerobic bacteria that grow on the filter media to break down waste more efficiently because the bacteria get exposed to more oxygen than if the media was submerged.

The water is then returned to a sump, where various other machinery may be housed, such as heaters, coolers, UV sterilisers, and protein skimmers, and even water level maintainers, before it is pumped back to the tank.

This type of filter is very common in marine fishkeeping, and also the modern "reef" tank type setup.

While it can prove very useful in freshwater aquaria, it is not seen as often.


Due to the value of nitrifying bacteria growing in one's filters, it is recommended to maintain at least two systems - identical ones that meet your needs are ideal - so that a complete cleaning of one, which might kill most of the bacteria, still leaves the other filter alive and healthy and ready to colonise the cleaned one. This does not apply so much to the wet/dry system, since the prefilter can be changed or cleaned without affecting the important trickle filter section.

External links[edit]