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In nature the water seeps through layers of earth and is thus mechanically cleaned. Microorganisms in the soil convert pollutants into nutrients. The water reaches from sources into streams, rivers and lakes. In these waters too, microorganisms ("cleansing bacteria") are responsible for the biodegradation of pollutants, such as fish discharges and dead plants. According to this principle also the cleaning of the water in the aquarium works over a filter.

The aquarium filter is therefore a very important component of an aquarium. It pumps off water from the aquarium, cleans it and then pumps it back into the aquarium. The filtration takes place in several steps, both by filter mass and by bacteria. The aquarium filter runs thereby around the clock to maintain the water conditions.

What kind of aquarium filter do I need?

A distinction is made between an internal filter and an external filter. Internal filters can be installed inside the aquarium because they are smaller than the external filters. This is also the reason why they are mostly used for smaller aquariums. External filters drain the water from the aquarium. This goes then via hoses to the filter, cleaned in several stages and then pumped back into the aquarium tank. Since external filters do not have to be installed in the aquarium basin, they can be larger in size and are therefore suitable for larger aquariums. So if you only have a small aquarium (<100l), you can buy an internal filter. The disadvantage is mainly in optics. For larger aquariums you can not get around buying an external filter.

How is an aquarium filter built up?

External filters consist of several layers. The soaked water will be filtered in several stages. The bottom layer consists of small ceramic filter tubes. They form useful space for bacteria settlement. The filter tubes are covered by a layer filter mat. A layer of filter substrate is placed over it.
This also provides useful space for bacteria settlement. This layer is covered by filter fleece. In addition, a layer of active carbon (in a net or in part also as a filter mat) can also form the last layer here. However, this should not be used permanently, but only after medication treatment and water turbidity caused by suspended matter.

When purchasing an aquarium filter or after a complete cleaning, the beneficial bacteria have to be replenished. Therefore exists special bacterial cultures which can be added to the aquarium filter and thus facilitate the start.

An overview of the best aquarium filters can be found on the following pages: