Biological Filter Media
Biological Filter Media is the material in the filter system that provides a home for aerobic bacteria. This bacteria is an essential part of the filter as, without it, it would be simply impossible to keep fish in an aquarium.
The Bacteria colony that exists within this media are responsible for a process known as the "Nitrite Cycle" where toxic ammonia generated from fish faeces & urea and their respiratory process and also organic decay is converted into Nitrite and then less toxic Nitrate.
The greater the surface area of the media, the greater the population of bacteria. More bacteria means greater processing power so a good quality media means the filter works more efficiently and can support a greater bio-load, i.e. more or larger fish.
Some filter media also provide conditions that favour anaerobic bacteria which may be used in de-nitrification or reduction of Nitrates.