The first few weeks of the new aquarium are crucial to its success. Before you think about buying fishes that will live inside, you need to prepare their future home. The aquarium is a miniature living world and to create that world it means to provide the conditions that will allow its main inhabitants live a healthy life in their small, closed world. Waste from the vital activity of the fish, uneaten food and all other organic matter that accumulates in the tank, can not disappear just like that, with a magic wand. They are converted by microorganisms and as the result of this process are toxic substances. There are creatures that eat exactly these substances as they transform into other compounds that can be used by other organisms. One of the most important compounds for us, which is a result of decomposition are based on nitrogen. The process in which they go through transformation called nitrogen cycle.
Who makes these transformations? They are made by microscopic creatures called denitrifying bacteria. Their role in the nature is to break down nitrogen compounds. When starting a new aquarium, these bacteria are existing, but they are not enough in the beginning. So it is very important during the first few weeks to make sure that this colony of bacteria is multiplying until it reaches the number of bacteria, sufficient to enable them to process the waste from fishes who will live in your tank. The life of our fish will depend on whether we can create a good colony of denitrifying bacteria. This process usually takes between 2 and 6 weeks.
Let us try to understand what the nitrogen cycle is. Nitrogen (N) is a chemical element that is a component of two important groups of organic molecules – proteins and nucleic acids. Although it is found in large amounts in the air, little creatures are able to absorb it in the form of gas (N2). Only one distinct class of bacteria, mainly Cianobacteria (often referred to as blue-green algae) can absorb N2, to use it for the synthesis of nitrogen-containing organic molecules. These bacteria are called nitrogen intermediaries. They are eaten by other organisms, which in turn becomes food for larger animals and so on, while the nitrogen compounds are spread throughout the ecosystem.
When these nitrogen compounds are released (death of the body or any part of it, or by its waste), they are processed by bacteria that decompose them and one of the main products is the ammonia gas (NH3). In contact with water, ammonia forms ammonium hydroxide (NH4OH), which in turn is a very toxic at high concentrations and has a strong corrosive effect. Ammonia is very harmful to the fish, the toxicity depends on the temperature, pH-and salinity of the water. For example, the more acidic the water is(with low pH), ammonium hydroxide is less toxic. A higher pH means more dangerous ammonia. Fortunately, this substance is absorbed without a problem by bacteria called Nitrosomonas, which in the presence of oxygen converts ammonia into nitrite (NO2-), thereby receiving energy:
2 NH3 + 3 O2 —-> 2 HNO2 + 2 H2O + energy
HNO2 (nitrous acid) is also dissolved in water by releasing one nitrite ion (NO2-). Nitrite is another highly toxic compound to plants and animals but fortunately it does not accumulate in the water because of the bacteria called Nitrobacter, it is transformed into nitrate (NO3-), thereby also releasing energy through the following reaction:
2 HNO2 + O2 —-> 2 HNO3 + Energy
Now the nitrogen, which began as part of the organic molecules, finally is becoming a much less toxic element. As a result of this whole process nitrates slowly start to build up in the tank. Of course, we should not allow them to accumulate too much because it can cause excessive growth of some microscopic algae that use them for food. To avoid this, you should make regular changes of the water and planting of plants in the aquarium, as they also use nitrates for food. In fact, the plants are good consumer of ammonia and are therefore very useful to keep the level of this toxin under control.
Nitrifying bacteria will attach to any place where there is enough oxygen (because the main part of the nitrogen cycle is aerobic, ie performed in the presence of oxygen). The colonies will thrive where there is not too much light and where water flow will not disturb them too much.
In terms of aquaristics maybe this is the most important part, the nitrogen cycle, but it does not end here. If the oxygen in the water decreases below a certain level, nitrates can be converted back to nitrite or (through a process called denitrification) can be transformed into nitrogen gas (N2) by anaerobic bacteria, and the cycle will be completed.
Now when you know how the nitrogen cycle works, we can better understand what to do in order for our new aquarium to provide a suitable environment for our fishes. The process of creating colonies of these bacteria is performed alone, without outside interference. All you need for bacteria is a source of organic matter. Once the tank is filled with water and filters are started, we need to add ammonia to start the cycle. Sometimes the tap water contains little ammonia and usually it is better to add amonia manually. Let me say once again – a good way to “start” an aquarium is to plant a vascular plant. Their metabolism and leaves that fall and decay, can provide the required initial nitrogen, while plants do not allow ammonia levels to rise too much. We can also add some dry food or a tiny piece of meat from fish or shrimp. There are also several commercial products designed to stimulate cycle on the market. Another good idea is to use a primer and/or water from an old aquarium, which already has established nitrogen cycle, and of course make sure that the source fish aquarium is “healthy.”
Another common practice is to use fishes to accelerate the process – put 2 or 3 healthy and unpretentious fishes that will live in the aquarium during the establishment of the cycle. This is not a good solution, because this way we are putting these fishes to unnecessary stress. The ideal solution is to buy a complete set of tests for freshwater aquarium (for pH, ammonia, nitrites, nitrates) and monitor the sequential increase and decrease of the levels of ammonia and nitrite (the ideal solution is quite expensive for our conditions but if you have patience and make necessary preparations after 2-3 weeks we can be sure that the levels of nitrite and ammonia have dropped to an acceptable level). When nitrite levels drop to zero once it was high, aquarium is ready to accept the fishes. But even then they should be put into it gradually to allow the bacteria to adapt to the increased amount of organic matter they have to deal with.