Ammonia, Nitrite and Nitrate

Ammonia is produced in the aquarium water as a result of naturally occuring processes. Fish excreta, urine and respiration all produce ammonia as do decaying plants and un-eaten food. In a mature, 'balanced' aquarium, the ammonia is converted into less harmful substances through the action of 'friendly' bacteria (known as Nitrosomonas) which convert the ammonia into Nitrite. Unfortunately, nitrite is only slightly less toxic than ammonia but a second group of bacteria (known as Nitrobacter) convert the nitrite into the much less toxic Nitrate.The nitrate is, in turn, used by the live plants as fertilizer - thus maintaining an equilibrium within the aquarium.

Nitrogen Cycle In a mature aquarium, both types of bacteria live in filter media, gravel and clean rocks but, in a new aquarium (or as a result of over-zealous cleaning of the filter media) the bacteria can be killed, thus destroying the natural balance. Only give the filter media a quick rinse in luke-warm dechlorinated water or discarded water from the aquarium. Water taken directly from the domestic supply will contain chlorine which may kill many of the beneficial bacteria.

Test kits are available to check the levels of Ammonia, Nitrite and Nitrate - typical examples are shown on the right. The significance of the ammonia test depends on the water's pH value and temperature as indicated in the following table. Test Strips

Maximum allowable Ammonia levels in mg/litre
Water
Temp (ºC) 		pH value
6.5 7.0 7.5 8.0 8.5 9.0
5 50 16.7 5.1 1.6 0.5 0.2
10 33.3 10.5 3.4 1.1 0.4 0.1
15 22.2 7.4 2.3 0.7 0.3 0.09
20 15.4 5.0 1.6 0.5 0.2 0.07
25 11.1 3.6 1.2 0.4 0.1 0.05

For the Nitrite test, levels below 0.1 mg/litre are acceptable for all species of fish, levels between 0.1 and 0.25 mg/litre indicate that there may be an excess of un-eaten food or decaying vegatation but is still acceptable to all but the most sensitive fish. 0.25 to 0.5 mg/litre is becoming unacceptable and above 0.5 mg/litre, immediate action should be taken to correct the level.

For high levels of ammonia and/or nitrite, the best solution is to carry out two 30% water changes with about 8 hours between them followed by a further 30% water change after another 12 hours.

The underlying cause of the high level(s) should be found and a careful check kept each day for a week. Thereafter, regular checks and 10% water changes about once every two weeks should be ok to take care of any possible build up of nitrate levels.