Nitrogen Cycle at the Aquarium 
Since the aquarium is a closed environment, the wastes generated by fish, food waste, fallen leaves of plants, etc. accumulate in it. This causes a high degree of toxicity in water, and although it looks clear, it will be lethal for our fish.
waste produced in the aquarium, are sources of nitrogen.
Nitrogen is mainly found in proteins. There are organisms that contain enzymes which act by breaking peptide bonds and resulting in elemental or amino acid components. Deamination of the latter by separation of the amino group, causes the ammonification, ie the release of ammonia.
Ammonia is the first substance formed during the nitrogen cycle and is the most toxic of them
Ammonia causes damage to gills, gills and kidneys. Decreases appetite and makes the fish more susceptible to disease. It causes bleeding and the best stress
total ammonia occurs in two forms: as ammonia (NH3) and as ammonium ion (NH4 +). The proportion of both depend on the pH and to a lesser degree of temperature. With an alkaline pH prodicirá more ammonia (very toxic) and an acidic pH more ammonia is formed (less toxic).
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In aquarium, ammonia is oxidized by bacteria to form nitrites.
2 NH3 + 3 O2 ---> 2 H2O + 2 HNO2
The bacteria responsible for this process are nitrite Nitrosomas.Los not have the toxicity of ammonia, but is still harmful to fish health
oxidation of nitrite to nitrate:
½ HNO2 + O2 ---> HNO3
here involved bacteria of the genus Nitrobacter.Como consequence of this process, nitrates accumulate slowly in the aquarium. It is important that your concentration is not excessive, although it is much less toxic than ammonia or nitrite, may in the long run harm the overall health of fish and also desorrollo algae in our tank, because these use them as food .
To prevent accumulation of excess nitrates are suitable partial and regular water and help a lot to introduce plants, because they feed them.
Bibliography: The nitrogen cycle of Hernán Ulrich
