Nitrogen Cycle

Ammonia and Ammonium.

Ammonia is largely produced by the breakdown of organic matter i.e. fish wastes and decaying plants. It can be present as toxic ammonia (NH₃), also described as ‘Free Ammonia’, and the relatively non-toxic form ammonium (NH₄⁺).

The actual proportion of each compound depends on temperature, salinity and most importantly, pH. A greater concentration of ammonia is present when the pH value is increased. Nearly 10 times more ammonia is present when the pH is 8.0 than when the pH is 7.0. At low pH levels, ammonia is more likely to appear as ammonium. Both types of ammonia are measured in test kits to arrive at the total concentration of ammonia in the water. Under ideal conditions, no detectable ammonia should be present in the aquarium. Levels over 0.02 mg per litre indicate a filtering deficiency. Certain species of fish tolerate ammonia better than others, but high levels of ammonia can create serious health problems and should be immediately corrected by a series of 30% water changes daily until levels become acceptable.

Other remedies include the use of ammonia absorbers or the addition of a fully matured filter. Your local aquarium supplier may have these available for hire.

Nitrite.

The ideal level of nitrite in any aquarium is zero. Ammonia is converted by Nitrosomonas bacteria as part of the nitrogen cycle and becomes less harmful nitrite. Though less deadly than ammonia it is still highly toxic to fish in high levels. You may experience a temporary increase in nitrite levels when you introduce new fish; this is due to the increased load on the filter. The friendly bacteria should soon multiply to high enough levels to deal with it efficiently.

Nitrite interferes with the oxygen metabolism, it destroys the haemoglobin (oxygen carrying cells) of the fish. Symptoms of nitrite poisoning include breathing problems and listlessness.

Nitrate.

Ammonia is converted by Nitrosomonas bacteria into nitrites, nitrites are then converted by Nitrobacter bacteria into less harmful nitrate. This is all part of the nitrogen cycle. Although nitrate is far less toxic than either ammonia or nitrite, it is still harmful to fish if the levels rise too high. Organic wastes from fish and plants increase nitrate levels. Other contributing factors are poorly maintained filters, over-feeding, over-stocking.

The normal filtration process will not remove nitrates and as a result they will build up over a period of time. The best way of controlling the levels of nitrate is to carry out regular partial water changes. This will dilute any nitrates present but, total elimination can be difficult without the use of a treatment to absorb it. Controlled quantities of nitrates have a useful role in the aquarium, they are absorbed as a good source of nutrients by healthy plants or corals and this helps to keep nitrate levels to a minimum.

The other method used to control nitrates is to use a deep sand bed commonly referred to as a DSB or otherwise a de-nitrate filter.