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What kind of filters can be used in the tank?

Two groups of filters exist - external and internal.

Internal filters clean the water by sucking it through the synthetic sponge material that fills a special container. The filtering material systematically gets clogged by debris and grains of substrate and needs to be rinsed. To do this the filter is regularly taken out the tank, disassembled and washed.

Internal power filters utilize an electrically powered air pump and are effective in small aquariums since have a small capacity and can hardly cope with very dirty water.

More effective are the external filters. They can be placed outside the tank - above it (Hang-on-Tank, HOT filters) or behind it (Hang-On-Back, HOB filters). The devices of the first kind are simpler and handier. The water comes up in the container along the tube, is drawn through the porous media and drops back into the tank. When the pores in the filtering material are locked, the water fills the box, overflows it and again gets inside the tank. This means the filter needs cleaning. For this purpose the rising tube is disconnected, the sponge media is taken out and rinsed. Thus there's no need to completely disassemble the system.

Sometimes large tanks are equipped with high-capacity external canister filters. They include the sections responsible for biological filtration and are called biological filters. After 1 to 2 months the surface of the sponge material becomes covered with bacteria, which take an active part in the tank matter cycle. Biological filters mechanically clean water from debris and also remove organic substances, humic acids, nitrites and nitrates. The filter must be rinsed approximately twice a year. The washed up filter begins to actively moderate the water biochemistry within about a month after the launch. Of course the mechanical cleaning begins from the moment the filter starts.

The undergravel filter is some kind of internal filter. To settle it, the false bottom or the system of drain pipes are organized.

Translated by Tatiana Karpova (Moscow)
(MSU, Biology faculture, Dep. zoology and ecology).