Aquarium Cycling: Everything You Need to Know

Written and verified by the biologist Cesar Paul Gonzalez Gonzalez

Last update: 22 December, 2022

Fish are famous for their dazzling colors and peculiar shapes, so it’s common for many people to keep them as pets. However, before bringing any fish home, it’s necessary to prepare the tank so that it gives them a good quality of life. For this reason, it’s important to carry out the cycling of the aquarium, which will help to establish the necessary microenvironment for the fish.

The natural habitat of these animals is made up of many species and each one helps to maintain the quality of the water to some extent. As a result, pollutants are partially cleaned out to maintain a healthy environment. Although it may seem difficult, this process can be simulated inside the aquarium by cycling. Read on and learn all about it.

What is cycling?

Within ecosystems there’s a mechanism for recycling waste, which purifies the entire environment and frees it from impurities. Although it isn’t perfect, the role of cycling in nature is important, because in this way the waste can be used by other living beings.

For example, when plants feed on organic matter in the soil, they are actually “ingesting” the remains and processed waste of different organisms. Therefore, the mechanism known as decomposition is nothing more than a set of reactions responsible for recycling nutrients from living things. As plants grow, they restart the trophic chain.

In the aquatic environment, this process is even more important, as waste is suspended in the water and can affect animals. For this reason, recycling mechanisms don’t only regulate the decomposition of matter, but also control the physicochemical parameters of the water.

As you can see, this process is actually a cycle that runs continuously in nature. In biology, all cycles are known as biochemical cycles and describe how each basic element is recycled from beginning to end. Cycling is the process of trying to simulate these mechanisms for the benefit of the fish in an aquarium.

Aquarium cycling

When it comes to a fish tank, cycling only serves to control the nitrogen suspended in the liquid. This is because it’s impossible to create each and every one of the biochemical cycles that exist in it, due to its small size. Moreover, this isn’t necessary, as regular water changes are necessary anyway to keep the aquarium hygienic.

The nitrogen cycle

In an aquarium, nitrogen is found in the form of ammonia, nitrites and nitrates, which can accumulate in the water. In general, ammonia is usually a product of the decomposition of any kind of organic matter, which, in this case, is the remains of food and fish waste. This compound is quite toxic and can cause the death of the fish.

In nature, there are bacteria such as Nitrosomonas which are responsible for transforming ammonia into nitrite, which is also a very toxic compound. However, nitrobacteria almost immediately transform nitrites into nitrates. Although these are still deadly compounds, living things can withstand a higher concentration of them before they’re harmed.

This is part of the nitrogen cycle, which allows beneficial bacteria to establish themselves in order to process and reduce toxic compounds in the water. The goal of an aquarist is to simulate this process, but, in this case, inside a tank. To do this, it’s only necessary to follow a few steps and be patient.

How to cycle the aquarium water?

The objective of aquarium cycling is to simulate the nitrogen cycle, as this will allow the transformation of ammonia into nitrates. To achieve this, the following steps must be taken before the fish arrive. First of all, set up the entire aquarium the way you want it to look. This should be your final setup, as we don’t recommend changing things afterwards. This includes the decorations, plants, substrate, filters and all accessories.

Once you are done, completely fill the tank and run the filters as if the fish were already inside. At this point, you’ll be trying to establish Nitrobacter and Nitrosomonas type bacteria so that they’re able to degrade the nitrogenous compounds. With this idea in mind, one of the following alternatives can be taken:

• Cycling without external help: This alternative is perhaps the one that takes the longest time, as it’s based on leaving the tank running without adding anything else. Although it sounds strange, the bacteria begin to establish themselves thanks to the water, the air, and the contamination that all the substrate brings with it. This process can take 2 months or more.
• Cycling with the help of ammonia (food or ammonia): As mentioned before, food remains are an important source of nitrogenous compounds, so adding them encourages bacteria to arrive to process them. In addition to this, you can also choose to add pure ammonia to the aquarium (only if you already have experience).

Ammonia-assisted cycling

When deciding to add ammonia on your own to speed up cycling, there are a few things to keep in mind. To begin with, the most commonly used option for adding nitrogen components is to use the feed in pellet or flake form. Only small portions should be added each day, which causes ammonia levels to rise and, soon after, the nitrite levels will also rise.

When using the feed to cycle the aquarium, the process usually takes between 3 weeks and 2 months in total. However, another option is to add pure ammonia in a controlled manner, which is the fastest route to cycling an aquarium. This last option requires precise control of the amounts, so it isn’t recommended if you’re a beginner.

Ammonia is added to the tank for the first 10 days at a rate of 4 milliliters per 100 liters of water (4 ppm concentration). If your aquarium has a capacity of 100 liters, you should only add 4 milliliters per day for 10 days. This process should be controlled by tests that measure the concentration of ammonia to ensure that it never exceeds 7 milligrams per liter.

Nitrate and nitrite concentrations should also be measured, as this allows you to see what’s happening during the process. As soon as the nitrites start to grow, it’s necessary to stop adding the ammonia, as this means that the bacteria have started to arrive. With this option, the aquarium cycling can be ready in just 20 days.

How do I know if my aquarium is ready?

At first sight, it’s difficult to determine that the aquarium has finished the cycling, because the clarification of the water is only one of the indicators. To have a better control of the process it’s advisable to use commercial tests that allow you to measure ammonia, nitrates and nitrites. With them, it’ll be easy to identify if your aquarium is ready to receive your new pet.

When a tank finishes its cycling, ammonia and nitrite measurements should be kept at 0, while nitrate measurements should be above 0. In other words, this means that the bacteria convert the ammonia to nitrates almost immediately; in this way, it’s the nitrates that accumulate.

What will happen to the nitrates?

As mentioned before, all nitrogenous compounds are toxic if they accumulate in the water. Although this includes nitrates, living things are a bit more resistant to them, so they can withstand higher concentrations of these compounds. However, this doesn’t mean you can’t deal with them, because if you’re careless, you can cause health problems for your fish.

In nature, these nitrates are used by plant roots. However, it’s sometimes difficult to put enough of them in the tank, so the best option is to remove them directly. This is achieved through water changes, because adding purified water dilutes the amount of harmful chemicals in the tank.

It’s true that this doesn’t completely reduce the concentration of nitrates, but it does keep their concentration stable in the aquarium. This is why water changes are so important, because without them the fish could die from poisoning in the tank.

In general, if the nitrate level exceeds 40 parts per million in the tests, a water change is necessary.

The cycling process is necessary in order to obtain a suitable ecosystem for your fish, so it must be carried out before introducing the specimens. Remember that, even though some species can withstand quite a bit without cycling, it isn’t right to put them through this long torture. Keep in mind that the health and quality of life of a pet is the most important thing.