Biological filters enable bacterial colonies to propagate and break down wastes. Biological filtration is the action of bacteria in the tank breaking down dangerous ammonia, converting them to nitrites, and then the nitrites to the less toxic nitrates. Nitrates can be harmful to many invertebrates in high concentrations. The process of biological filtration, also known as the nitrogen cycle, involves the movement of harmful wastes through the filter. Biological filters should be checked regularly to ensure particles have not escaped the mechanical filters, clogging the biological filter and decreasing efficiency.

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