Different to the UV filters which effectively used to bacteria, biological filters functions instead provide a place for the growth of bacteria to through a cycling process, convert a harmful pollutants in water to harmless nitrate. Nitrate is nitrogen, therefore this cycling process is also referred to as the nitrogen cycle, or cycling a tank.
Beside live in the biological filters the good bacteria in fact also grow in the aquarium, such as on sand, rocks, and even water plants. However, this amount is not enough to break down ammonia and nitrite are formed in the aquarium. Therefore must be provided a bigger place for growth, namely in the filter media.
Given the bacteria requires a lot of oxygen for life and growth, the availability of oxygen all the time is absolute. This can be achieved if the water flow through the filter media is strong enough, so that oxygen can be numerous, or we can used biological filters with filter media that many cavity and not quickly clogged, for example bioball.
