Aerobic biological filtration for freshwater aquariums


Biological illustration


Aquarium technicians and home enthusiasts


Ammonia (NH3) is a waste product excreted by fish and the decomposition of uneaten food. NH3 is toxic to fish and must be continuously filtered out of a closed system like an aquarium. Fortunately, microorganisms called nitrifying bacteria can consume NH3 as their source of energy, eventually converting it to a non-toxic substance called nitrate (NO3).

This set of illustrations depicts three different designs for aerobic biological filtration of nitrogenous waste in aquariums. Water passes through a high surface area medium (ie: gravel, bio ball, or sponge) that permits colonization of nitrifying bacteria. Oxygen is diffused into the water by either agitation (biotower filter) or pumping room air through an air stone (under gravel or sponge filter).



For more information on the nitrogen cycle in aquariums,
check out the online resources listed below:

Florida Department of Agriculture and Consumer Services:
Aquarium water quality: Nitrogen cycle

The aquarium co-op:
The easy guide to the nitrogen cycle for aquariums

Aquarium nitrogen cycle – new tank syndrome


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