This works like the air filter and oil filter on a car, or the filters in your furnace. This removes harmful substances such as ammonia and carbon dioxide, both of which are by-products of the life process. It also removes poisons accidently introduced to the aquarium, such as cigarette smoke and household cleaning chemicals. Biological filtration requires the use of beneficial bacteria. That's why The Fish Guy never cleans the inside of those tubes or the interior of filters.
Beneficial bacteria is also present in the gravel.
Biological filtration is arguably the most important of the three. Pollutants naturally occurring in the aquarium, such as fish waste and rotting bits of uneaten food, can kill aquarium fish. The beneficial bacteria render these pollutants harmless. This process is called the Nitrogen Cycle. The beneficial bacteria are known as nitrifying bacteria.