Eheim Ehfisubstrat Pro Filter Media

A build-up of toxic nitrogenous wastes is a natural result of all aquarium inhabitant's life processes. In nature, the body of water is large enough to dissipate or dilute these wastes. Some will even be converted into usable energy by living organisms. Within an aquarium however, without a powerful biological breakdown of these toxins, the fish literally poison themselves. This is the reason why biological filtration is the most desirable of all aquarium filtration.