The Nitrogen Cycle for an Aquarium

The Nitrogen Cycle for an Aquarium

The first weeks of a new aquarium are crucial to its success. Before we think about buying fish to add to our tanks, we must first prepare the home for them. An aquarium is a small miniature living world, and preparing the house means to establish this little world all the necessary biology, which will enable healthy living in a small and enclosed place. Faeces, not consumed food, the fish waste and other organic matter that accumulates in the aquarium do not disappear with magic. They are decomposed by microorganisms, often resulting in toxic substances. But as nature is wise, there are beings who want nothing more than turning the matter decomposed into other compounds that may be used by other beings. One of the most important classes of compounds that result from decomposition is the nitrogen, and the process by which it is gradually transformed is called the Nitrogen Cycle.

How and who makes these changes? Microscopic beings that are called nitrifying bacteria, whose function in nature is decomposers of nitrogen compounds. When we set up a new aquarium, these bacteria exist only in very small quantities (those few who happen to come with the water, gravel, etc.). It is therefore essential, in the first week, making this colony of bacteria multiply until it reaches an amount that is capable of processing the fish waste that will follow. Thus, depend on the formation of a good colony of nitrifying bacteria to ensure a healthy life in our aquarium. In the language of the aquarium, this initial period of colony formation is often called cycling the tank. An aquarium will be ready to receive the main population of fish when it is properly cycled. This process normally takes between 2 and 6 weeks to complete.

Let's understand better this cycle. Nitrogen (N) is a chemical element that enters into the formation of two important classes of organic molecules, proteins and nucleic acids. Although present in large quantities in the air, in the form of nitrogen gas (N2), few living beings can assimilate this form. Only certain types of bacteria, especially cyanobacteria (formerly known as blue algae) are able to capture the N2, using it in the synthesis of organic nitrogen molecules. These bacteria are called nitrogen fixers. They end up being eaten by other organisms, which in turn are eaten by other animals, and so on until the nitrogen compounds are spread over all living beings.

When these nitrogen compounds are released (by the death of an organism or their excretions), they are processed by decomposing bacteria, and one of the main products of this decomposition is Ammonia Gas (NH3). The ammonia in contact with water, form Ammonium Hydroxide (NH4OH), a highly toxic substance which in large concentrations is a highly corrosive. Ammonia is a very dangerous substance for fish, and its toxicity depends on temperature, pH and salinity. For example, the more acid pH, more Ammonium Hydroxide is neutralized and therefore decreases the toxicity of ammonia. On the other hand, more alkaline pH is more dangerous Ammonia. Fortunately, this substance is consumed by bacteria called Nitrosomonas, which in the presence of oxygen transforms the ammonia into nitrite (NO2-) obtaining energy through the following process:

2 NH3 + 3 O2 ----> 2 HNO2 + 2 H2O + Energy

The HNO2 (nitrous acid) dissolves in water releasing the nitrite ion (NO2-). Nitrite is another highly toxic substance for plants and animals, but fortunately it does not accumulate in a well set up tank, because bacteria from the Nitrospira convert into nitrates (NO3-), also obtaining energy through the reaction:

2 HNO2 + O2 ----> 2 HNO3 + Energy

Now yes, our nitrogen which started decomposing in organic molecules finally took a much less toxic form. In the aquarium, nitrate will slowly accumulate as a result of this process. But we must not let it accumulate much because it may lead to excessive growth of algae which use as a nutrient. To avoid this, we do regular water changes and better yet, add natural plants in the aquarium, because nitrate is readily consumed by them. In fact, plants are also good consumers of ammonia, and therefore very helpful in keeping this toxin under control.

The nitrifying bacteria will settle in any location whith a good supply of oxygen (as the main process of the cycle is aerobic, ie, in the presence of oxygen). However, the colonies will prosper in places without much light, and where the stream does not disturb them too much. This is the most important part of the Nitrogen Cycle in terms of fishkeeping, but in fact it does not stop there. For example, if lacking oxygen the nitrate can be transformed back into nitrite or, through a process called denitrification, it is again transformed by anaerobic bacteria into nitrogen gas (N2), and the cycle is complete.

Now that we know the Nitrogen Cycle, we can understand better how to proceed in a new tank to ensure a healthy environment for fish. The process of colonization of bacteria occurs without the need to intervene until there is a source of organic matter. Once the water is inserted into the aquarium and connected the filters, we need to provide a bit of ammonia to start the cycling process. Sometimes the tap water itself already contains ammonia, but in general it is better to encourage the process. Again, a great way to start is by adding natural plants. Their own metabolism and the falling leaves provide the initial nitrogen, as already mentioned they help keep the ammonia level from getting too high. You can also put a pinch of food, or a small piece of fish or shrimp, and there are commercial products to stimulate the cycle. Another good procedure is to use a bit of gravel and/or water from an aquarium already matured.

It is very common to use "cycling fish" to accelerate the process. Adds 2 or 3 hardy fish (zebrafish, for example) to live in the tank while it is passing through cycling. But this is not a good solution because you're submitting these fish to unnecessary stress. The ideal is to buy a test kit full of fresh water (pH, ammonia, nitrite, nitrate) and follow the ups and downs of ammonia and nitrite. When the nitrite drops to zero after rising, the tank is ready to begin the colonization of fish. But even so the population should be increased gradually to allow the amount of bacteria will also adapting to the increased biological load.

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