Eheim substrat pro is a biological mediar this type of filtration is the purification of the aquarium water by using living organisms such as denitrifying bacteriathese desirable bacteria will attach themselves to all hard surfaces within the aquarium systemthey use the toxins as a food converting harmful toxins such as ammonia and nitrite into less harmful nitrateafter prolonged use mechanical and chemical filter media will act in a biological manner they provide some surface area for b