Filters under this category cater to proper and smooth functioning of Nitrogen Cycle. The proper management of the nitrogen cycle is a vital element of a successful aquarium. Metabolic wastes and excreta and other decomposing organic matter produce ammonia which is highly toxic to fish. The primary function of Biological Filter is to ensure proper breakdown of dangerous substances to make it suitable and conducive for aquarium ecology. (For details refer to Nitrogen Cycle)
In most cases, a biological filter is nothing more than a chemically inert porous sponge, which provides a greatly enlarged surface area on which these bacteria can develop.