THE NITROGEN CYCLE

After the tank is set up with a running filter, the fishes are added to the aquarium. At this stage, your filter is merely a gadget that circulates the aquarium water to provide the fishes with oxygen. However, your filter becomes barren and unable to process your aquarium water which, after a few days, becomes polluted. It will take about 30 days for your filter to function properly by developing a healthy population of beneficial bacteria, which is responsible for completing the nitrogen cycle and keeping your aquarium clean by processing the poisonous waste products.

The first stage of the nitrogen cycle starts once we have set up the aquarium. When we give food to the fishes, they will naturally process the food and release excrements into the water. Fish waste, like feces, uneaten food, decaying plants, and other organic matter will rot and become ammonia. Ammonia mixed in aquarium water is highly toxic to fishes and can kill them.

Ammonia then accumulates in the tank and your filter will develop certain species of beneficial bacteria to start the biological filtration process. The Nitrosomonas bacteria lives in the filter media and consume toxic ammonia. Nitrites, a less toxic substance, will be released as a result.

Though generally less toxic, nitrites will increase in amount and signal the next stage of the nitrogen cycle as other species of beneficial bacteria develop in the filter media. The Nitrobacter bacteria then consume the nitrites. A new, less toxic substance is created. The byproduct of this assimilation is nitrate.

Once we have developed a good colony of Nitrobacter in our filter media to convert nitrites to nitrates, we can conclude that we have undergone the different stages of the nitrogen cycle. It is at this point that we can say we have achieved a “cycled tank”. Our filter is now biologically functional and can process the highly poisonous ammonia to less toxic substances -- nitrites and nitrates.

While nitrate is the least toxic among the three substances, it is nevertheless still poisonous to fishes at very high levels. Therefore, when the aquarium has completed its cycle, nitrates slowly accumulate. If the aquarium has live plants, they will consume the nitrates, but if not, we must compensate by conducting partial water changes. Dilute the nitrate to safer levels by removing about 30 percent of old tank water and replacing it with fresh water.

In doing so, water quality is maintained. With that, the environment for fishes is of optimum conditions and they are able to thrive in the aquarium.