NITROGEN CYCLE

The nitrogen cycle is the natural process that occurs in your tank to transform the harmful chemicals produced by the fish via excrement etc. into less harmful chemicals. Out of all the cycles in your tank it is one of the most important to understand and any serious fish keeper should realise the impact it has on your aquarium. This cycle is also responsible for New Tank Syndrome and is very important in tank cycling, which is covered in another sheet.

The Nitrogen Cycle involves three main chemicals; ammonia, nitrite and nitrate. During the cycle, these chemicals are transformed into less harmful chemicals by bacteria and plants. Due to the difficulty of detecting ammonia in the past and how little was known of its affect on the cycle this process was known as the Nitrite Cycle at one point.

The nitrogen cycle runs as follows (whilst reading through these instructions it may be helpful to refer to the diagram opposite):

Ammonia [NH3] enters the water through fish respiration and excretion, as well as decaying matter in the tank.

Ammonia is toxic to fish and normal symptoms for ammonia poisoning are ragged fins and inflamed/red gills. Even low amounts can cause illness so always ensure than your ammonia readings for your water are zero.

Plants use ammonia as a food source, although they use very little.

Ammonia is converted into nitrite [NO2] by nitrosomonas bacteria (as well as others).

Nitrite is less toxic to fish than ammonia, but is still harmful. Low levels can be tolerated by fish, but always ensure than any readings you take are zero. If you have a constant nitrite reading then there is a problem in your system.

Nitrosomonas and nitrobacter - see next step - bacteria are known as litho-tropic bacteria. This means that they require oxygen to survive.

Nitrite is converted into nitrate [NO3] by nitrobacter bacteria (as well as others).

Although not nearly as toxic as ammonia and nitrite, high nitrate levels can lead to outbreaks of disease such as white spot and fungus due to the nitrates reducing the fish's resistance to disease.

High nitrate levels may also lead to algal bursts as the extra nutrients provided by the nitrates promote their growth.

Nitrates are converted into harmless chemicals by plants and algae

Although used by plants and algae as a food source, under normal conditions - unless an aquarium is well stocked with healthy plants - nitrates will build up over time.

Regular water changes will help to reduce the amount of nitrate, although some local water supplies may already have high levels of nitrate prior to making a water change.

Adding extra plants to an aquarium will help to reduce nitrate as well as keeping algae growth down. This is due to the plants out-competing the algae for nutrients.