Biological filter

The heart of the filtration system. It hosts bacteria that make the nitrogen cycle possible by transforming toxic substances into less dangerous compounds.

Detailed explanation

The biological filter does not filter anything in a mechanical sense. Its job is to provide a huge surface area where nitrifying bacteria can colonise and work.

The most used biological filter materials are ceramic rings, bioballs, pumice, sintered glass bricks (such as Siporax) and large-pored sponges. The choice of material matters up to a certain point: what really matters is the total available surface area and the flow of water through the material.

A common mistake is washing the filter material under the tap. Chlorine in drinking water kills bacteria in seconds. It should always be rinsed with aquarium water during changes.

In marine aquariums the traditional biological filter loses importance because live rock perform the same function. Many modern reef aquariums do not have biological filters in the classic sense: they only use live rock in the display or sump.

In tropical freshwater aquariums the biological filter is irreplaceable. External canister filters offer the best results because they have a lot of volume for the filter material and a constant flow. Internal filters work for small aquariums but saturate quickly.

For ponds, chamber filters with very generous volumes of biological material are used. The general rule is that the filter volume should be at least 10% of the total pond volume.

How it is measured

Not measured directly. The efficiency of the biological filter is evaluated by monitoring ammonia and nitrites: if they remain at zero the filter works well.

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