What is the nitrogen cycle in an aquarium?

The nitrogen cycle is an extremely important biological process on the reef, and in all ecosystems, for that matter. It is an important purification process, however it is more than that, it is also the source of primary food production.  Without this citical process nothing on earth would survive.

Disturbances to this process, caused by pollution from human waste and chemicals, are putting a serious question mark over the future of our world.  This is because in many places not even the vast exchange of ocean water is capable of diluting these substances. Likewise, nothing will survive in the aquarium without proper utilisation of the processes within the nitrogen cycle.

With aquariums, the nitrogen cycle is known as ‘cycling’ your tank.  This is effectively the process of building up beneficial bacteria to break down harmful ammonia and nitrites to result in good conditions for plant and fish life.

The nitrogen cycle is basically the removal of nitrogenous compounds that gradually build up in the aquarium water.  Starting with free ammonia (NI-I3) and ionized ammonia (NH4). Fishes and invertebrates produce ammonia as a waste product from their metabolic processes. This is added to the ammonia produced by bacteria interacting with other waste materials in the aquarium.  Examples of these are uneaten food and faeces.

Ammonia is highly toxic to fishes and invertebrates.  If ammonia is not removed, or converted into other less harmful substances, your aquarium subjects will soon perish. Fortunately, in Nature’s Grand Plan, a substance that is poison to one living organism is food for another, thus, there is a natural way of dealing with ammonia removal.

Aerobic (oxygen-loving) bacteria, such as Nitrosomonas species, convert ammonia to nitrite (N02).  Nitrite is a slightly less toxic substance but one that is still dangerous to fishes and invertebrates. A second group of bacteria, such as Nitrobacter species, transform the nitrite to nitrate (N03).  Nitrates are much safer substances but one that can still cause some problems if allowed to build up too much. It is not fully understood to what level nitrate can cause problems for aquarium subjects.  In high volumes, nitrate does appear to be detrimental to delicate fish species and some invertebrates. The important thing is that nitrate does not occur on the natural coral reef other than in the smallest quantities.  As a result, we should not make compromises but strive to keep it to an absolute minimum in the aquarium.

Nitrate is a primary nutrient for algae and the mainstay of primary food production right at the beginning of the food chain.  The food chain from which all other living organisms benefit. It is worth noting that many species of undesirable algae may flourish with the presence of excess nitrates, as well as phosphates. By the process of denitrification, anaerobic, or oxygen-hating, bacteria convert nitrate into free atmospheric nitrogen, which is naturally vented from the aquarium.

The conditions that allow this are often found in porous material such as ‘living rock’, where pockets of oxygen-deficient water can be trapped, promoting the growth of anaerobic bacteria. The same thing can happen in the bottom levels of substrate, but here the gases can get trapped, including potentially dangerous sulphur (the gas with a pungent smell like bad eggs). However, herein lies a paradox, since if we are to provide ideal conditions for the complete removal of nitrogenous compounds, then it appears we must provide two sets of conditions for opposite types of bacteria.

There are two methods for starting up a new tank.  One is with fish the other is without fish. Without fish is the preferred method as less hardy fish can die through the first stages. With either method is it critical that you have a water testing kit. This will allow you to monitor the levels of ammonia, nitrites, and nitrates in the water.

Firstly and by far the most important part is to understand the nitrogen cycle. You need to allow the cycle to initially run to break toxic nitrogen waist down into components that are less harmful.  Effectively, over time ammonia will break down to nitrites.  Then the nitrites will break down to nitrates.

A partial water change can help reduce ammonia and nitrite levels in your water. This is a process where you take up to 20% of your water out and replace it with new water. Also, live plants in freshwater and live rock and deep sand in saltwater will help break down levels. Whatever method you are using you need to ensure that ammonia and nitrite levels are near zero.

Fishless cycling would be my preferred method but does require patience to allow the cycle to run. First of all, start by adding fish food to the tank.

After a few days, as the food starts to decompose, test the water for ammonia.  Keep ammonia levels at about 3ppm.  If this drops, then put some more food into the tank.  After a week, start testing for nitrite readings.  Once you start getting nitrite readings this indicates that the nitrogen cycle has begun.

Keep feeding the tank and nitrites will continue to rise.  Keep testing for both nitrites and nitrates.  As a result, you should see a sudden drop in nitrites and an increase in nitrates which indicates the final phase of cycling. Consequently, ammonia and nitrite levels will reduce to near zero levels.

 When this happens, and nitrate levels maintain a consistent level is safe to start adding your fish.