Fishkeeping is a very interesting hobby that many Filipinos take after. However, it has its own set of challenges.

From setting up the tank to keeping the water in pristine conditions, there are many things to learn before one is able to successfully keep an aquarium.

Among those, the most important basic principle in aquarium fishkeeping is maintaining its “nitrogen cycle”. Understanding the concept will spell the difference between having a successful aquarium and a failed attempt.

“Cycling a tank” is what every fishkeeper should wish to achieve. This is basically completing the different stages of the nitrogen cycle so that the filter system maintains good water quality in the aquarium.

The aquarium

One thing that sets fishkeeping apart from other pet keeping hobbies is its unique environment. Dogs, cats, birds, reptiles, rabbits, hamsters, and other pets live on land and basically live the same way as we humans do. Just provide the same environment as ours and they will be generally fine. For example, a birdcage inside your house should provide ample environment that is livable enough for your pet bird.

Since fishes live in water, you will have to recreate this inside the confines of an aquarium. It is not merely keeping water in the tank, but also providing a filter system that sustains good water quality. Unfortunately, fishes do not have bathrooms where they can defecate and urinate as we humans do. They just release their biological wastes in the very same water they live in. Fishes are highly dependent on the quality of water they live in, so if the water is polluted, they can get sick or, worse, die.

The aquarium is really very restrictive. Considering that fishes live in vast rivers, lakes, and seas, an aquarium, no matter how big, is minuscule compared to a fish’s natural environment. With limited space, aquarium water will quickly get polluted once the fish produces waste.

Unfortunately, setting up a filter system in your aquarium will not automatically effectively and efficiently clean the water. Your filter will have to be biologically active for it to function correctly. This is where the nitrogen cycle comes in.

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.

Biological filtration

The nitrogen cycle is the very essence of biological filtration, which keeps aquarium water clean. To achieve effective biological filtration, you need to have a viable population of beneficial bacteria in your filter media, organic matter, and oxygen. All these are present in a correctly set up aquarium.

Whatever filter system you will be using, you will be assured it will have filter media, which serves as the home of beneficial bacteria. Organic matter will be present in the aquarium once you have fishes and other living things, and they will serve as food for the beneficial bacteria. Lastly, oxygen will be supplied by your filter by creating agitation at the surface of the water. Beneficial bacteria are aerobic and therefore need oxygen to live. A correctly set up tank will have all four factors, leading to the full completion of the nitrogen cycle in about 30 days. The filter system should process ammonia and nitrites into the safest substance, which is nitrates.

Achieving a cycled tank has its challenges. Others fail to remember a newly set up tank is not yet capable of sustaining underwater life, since the filter is not biologically active yet.

The term “New Tank Syndrome” (NTS) is a fishkeeper’s nightmare. It occurs when fishes are fed in a newly set up tank incapable of processing the toxic substances. Ammonia quickly increases to levels that are too high for fishes. Since the filter has not developed a viable colony of Nitrosomonas that can consume the ammonia, the tank becomes poisonous to fishes. The more we feed the fishes, the more waste they produce and the higher the ammonia levels become.

To avoid NTS and increase the chances of a successful nitrogen cycle, it is recommended to start a tank with a few fishes to be fed once every other day for the next 30 days. This way, you will have less ammonia while your filter isn’t biologically active yet.

Important reminder

You will still need some fishes in your newly set up tank, because their waste is one of the factors that completes the nitrogen cycle. Be patient and wait for the next 30 days. After your tank has cycled, you can add more fishes and feed more often, and reward yourself with a functional aquarium.

Also, proper filtration alone cannot guarantee clean aquarium water. Biological filtration keeps water clean, but over time, nitrates will eventually accumulate to levels that are poisonous to fishes. Therefore, proper maintenance is needed to keep tank water in optimum conditions. Proper filtration and periodic maintenance are the keys to keeping water in best condition for our fishes.

For comments, suggestions, and questions, write Angel Ampil at

This appeared in Animal Scene magazine’s June 2019 issue.

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