This type of filtration is the purification of the aquarium water by using living organisms, such as denitrifying bacteria. These desirable bacteria will attach themselves to all hard surfaces within the aquarium system. They use the toxins as a food, converting harmful toxins such as ammonia and nitrite into less harmful nitrate. After prolonged use, mechanical and chemical filter media will act in a biological manner. They provide some surface area for bacterial colonization but this is actually very limited. In order to provide the greatest amount of surface area for bacteria colonies, filter media such as Eheim Substrat Pro should be used.