Nitrogen cycle

The biological process that transforms toxic fish waste into less dangerous substances. Without a mature nitrogen cycle no aquarium can function.

Detailed explanation

The nitrogen cycle is the basis of everything. Fish produce ammonia through their gills and excrement. Ammonia is toxic even at very low concentrations. If it is not transformed the fish die.

Nitrosomonas bacteria (and other similar genera) colonise the porous surfaces of the biological filter and convert ammonia into nitrite. Nitrites are still toxic but a little less than ammonia.

A second group of bacteria, Nitrospira, takes nitrites and transforms them into nitrates. Nitrates are much less toxic and are removed with water changes or consumed by plants and algae.

In a new aquarium the cycle takes from 3 to 6 weeks to fully stabilise. No fish should be added during this period. Many beginners skip this step with disastrous results.

In marine aquariums the concept is identical but the live rock act as the main substrate for the bacteria. In very mature aquariums, denitrification also develops: anaerobic bacteria in the internal areas of the rocks convert nitrates into nitrogen gas which evaporates.

In ponds the cycle works in the same way but the larger volumes and the presence of aquatic plants help absorb excess nitrates. The problem comes in spring when the fish start eating again but the filter bacteria are still slowed down by the winter cold.

How it is measured

It is monitored indirectly by testing ammonia, nitrites and nitrates. During maturation, tests are carried out every 2-3 days. The cycle is complete when ammonia and nitrites remain at zero with nitrates increasing slightly.

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