Aquarium filtration

A filter does two jobs in a tank. The mechanical part, sponge and floss, traps floating dirt, while the biological part houses bacteria that turn ammonia and nitrite, both toxic to fish, into less harmful nitrate. That bacterial colony is what keeps the water healthy rather than merely clear.

An internal filter is easy to fit and suits a small tank; a sponge filter suits shrimp and fry. An external filter sits in the cabinet, holds more media and leaves the inside of the tank free for plants. A UV steriliser helps against green water, but it does not do the bacteria's work.

The most common mistake is cleaning the filter too thoroughly. Rinse sponges in tank water, not tap water, and never replace all the media at once, or the colony is lost and the water can turn within days.