This system breaks down any toxic compounds, usually Nitrogenous toxins. The Nitrogen Cycle
Biological filtration is always the primary filtration. Biological filtration converts highly toxic waste, which are produced by the animals in the aquarium system, into less toxic nutrients through the activity of nitrifying bacteria.
These filters make use of bacteria to break down wastes in the water into substances which are less toxic to the inhabitants, in a process known as the nitrogen cycle. Examples are under gravel filters, sponge filters, and trickle filters.