What Is the Nitrogen Cycle in an Aquarium?

The nitrogen cycle is a natural process that can keep your aquarium safe for fish. It can work the same way for both saltwater and freshwater aquariums. During the process, different types of beneficial bacteria develop and perform different jobs. Each type of bacteria has its own job to do. The first group is responsible for converting ammonia into nitrite. The second group converts nitrite into nitrate. These three substances are known as the main stages of the nitrogen cycle. 

Ammonia is very toxic to fish. Nitrite is also toxic, but it can be produced when ammonia breaks down. Nitrate is much less harmful, and it can be removed with water changes. Even live plants absorb a lot of nitrate. 

A new aquarium can take about 4-6 weeks to complete the nitrogen cycle. However, the exact time can vary. Some of the factors, such as the number of fish, tank size, the filter, and beneficial bacteria in the aquarium, can all affect the cycle. That's why it is important to test the aquarium water regularly. A water test can help you know how much ammonia, nitrite, and nitrate are present. You can actually know which stage of the cycle your aquarium is in. 

Check out How to Test Aquarium Water (pH, NH3, NO2, NO3).

If you want to improve the process, you can even use methods such as adding beneficial bacteria. You also have an option to use a filter media from a healthy aquarium. These techniques can help reduce cycling time, but you should still test the water to make sure the nitrogen cycle is fully established before you add more fish. 

What Are The Three Components and Phases?

Ammonia

The first stage of the nitrogen cycle starts with ammonia. It is the most harmful waste product in an aquarium. Ammonia is produced when fish release waste, uneaten fish food breaks down, and dead plant leaves or other organic matter decay. When ammonia starts building up, the nitrogen cycle begins. Even a small amount of ammonia can be toxic to fish. High ammonia levels can cause damage to their gills and make it difficult for them to breathe. If not treated, it can be fatal. During the cycling process, it is normal for ammonia levels to rise because beneficial bacteria have not yet grown in large enough numbers to remove it. 

Even if ammonia is harmful, it is also essential for starting the nitrogen cycle. Beneficial bacteria usually feed on ammonia and use it as their food source. When these bacteria multiply, they convert ammonia into nitrite, moving the cycle to the next stage. If you remove all the ammonia too early by constantly changing the water or using ammonia-removing products, the beneficial bacteria do not get enough food to grow. It can even slow down or interrupt the cycling process. 

Ammonia Toxicity

The amount of harm ammonia causes depends on your aquarium's pH and water temperature. If there is a higher pH, it makes ammonia more toxic. However, warm water also increases ammonia's toxicity. That's why it is important to test your aquarium water regularly when cycling. Checking ammonia levels helps you track the progress of the nitrogen cycle and ensures your aquarium becomes safe for fish.

Here is Why Regular Aquarium Water Testing Is Crucial for Your Fish’s Health.

Fishless Cycling

Instead of exposing fish to harmful ammonia levels, most aquarium owners choose fishless cycling. Ammonia is intentionally added to feed the bacteria without adding fish. Removing all of it too quickly can slow bacterial growth. However, if fish are already in the aquarium and ammonia reaches dangerous levels, partial water changes are needed to protect them.

Good Bacteria for Aquariums – A Comprehensive Guide for Aquarium Health.

Nitrite

About 1-2 weeks after the nitrogen cycle starts, a group of beneficial bacteria known as Nitrosomonas begins to grow. These bacteria feed on ammonia and then convert it into nitrite. However, Nitrospira (or Nitrobacter, depending on the system) converts nitrite into nitrate. Even if it is an important step, nitrite is still toxic to fish. Low levels can cause stress and health problems, so they should not remain in the aquarium for long. During that stage, ammonia levels start to decrease as the bacteria consume it. Sometimes nitrite levels rise and even reach high levels. It is usually the most critical part of the cycling process because nitrite is still harmful. 

As more helpful bacteria develop, another group of bacteria starts converting nitrite into nitrate. Once it happens, nitrite levels begin to fall. In most aquariums, nitrite drops to zero by the end of the cycling process. However, every aquarium is different, so some tanks can take a little longer. Regular water testing is the right way to track your progress and know when your aquarium is fully cycled. 

Nitrate

The final stage of the nitrogen cycle starts when another group of beneficial bacteria begins to grow. These bacteria convert nitrite into nitrate. It is much less harmful to fish. Like other helpful bacteria, they need oxygen and a steady food source to live. They grow on surfaces throughout the aquarium that also include gravel, filter media, decorations, and glass. As these bacteria become established, nitrite levels can drop to zero, nitrate levels start to rise, and it is a sign that your aquarium's nitrogen cycle is working properly. 

Nitrate is much less toxic than ammonia and nitrite. However, it can still become harmful if it builds up over time. If you want to keep nitrate levels under control, do regular partial water changes, and if you grow plants, let them absorb some of the nitrate as a nutrient. When ammonia and nitrite both test at zero, and nitrate is present, your aquarium is considered fully cycled and ready to safely provide support to the fish.

Learn How to Do a Water Change Without Stressing Fish.