What is the filter

A filter is a device for removing floating particles of the larger kind from aquarium water and even for absorbing some of the gases therein.

It is usual to connect the air pump to an 'air lift' by means of which the aquarium water can be pumped back into the tank after it has been siphoned from it and passed through the filter.

The filter normally consists of three layers; the first - such as the fine sand, spun glass or cotton wool. Their functions are to remove floating particles. The second layer is the activated carbon which is the core of the apparatus which even removes the gases and some of the organic compounds such as toxic products due to the presence of fish in the water. The third layer is merely a coarse layer of small stones allowing the filtered water to percolate to the bottom of the 'air lift' through which it is pumped back into the aquarium.