Resource Center Menu
Nitrogen Cycle

Nitrogen Cycle

Clean water is as vital to fish as air is to land animals. Pet fish need clean, filtered water to survive. To understand what is required to keep your fishes’ environment healthy, you need to understand the nitrogen cycle in aquariums, which is sometimes referred to as biological filtration.

The nitrogen cycle

The nitrogen cycle is a chain of biological reactions that produces chemical results. It begins when decaying food and fish waste produce ammonia. Ammonia is very toxic to fish and, in the small confines of an aquarium, can eventually kill them. Fortunately, however, ammonia is "food" for nitrifying bacteria, which are present in water.

The nitrifying bacteria "eat" the toxic ammonia, producing nitrite. Another strain of nitrifying bacteria "eats" the toxic nitrite, producing the less toxic byproduct nitrate. Since nitrate is relatively harmless to fish unless it accumulates in large quantities, the toxic effects of the ammonia and nitrite are canceled out by the biological food chain. To keep nitrate levels safe for your aquatic life, you’ll need to filter your aquarium water and change a portion of it regularly. The nitrogen cycle is what keeps the chemical balance of water at life-sustainable levels for plants and fish. It is important to test your aquarium's water regularly and make necessary adjustments by completing a water change and adding or replacing activated carbon, ammonia neutralizers or water conditioners as your water test levels indicate.

Using the nitrogen cycle to prepare an aquarium

Even with the addition of bacteria supplements, the nitrogen cycle can take between six and seven weeks to complete and stabilize. The chart below shows how the cycle works and the approximate time before ammonia is oxidized into nitrite and the nitrite is oxidized into nitrate in both freshwater and saltwater environments. After the first six to seven weeks of setting up an aquarium, the tank's nitrogen cycle should be stabilized. During those first few weeks of setting up a new aquarium, only a few hardy fish should be employed to initiate the nitrogen cycle. The water must be monitored closely for the following readings:

  • During the first two weeks, ammonia should be kept under .06 ppm (parts per million). Water should be tested every two days until ammonia is controlled.
  • After two weeks, ammonia levels should read .0 ppm and nitrites should increase and be maintained below .75 ppm.
  • After the fifth or sixth week, nitrites should decrease and nitrates should increase, approaching a reading of 25.0 ppm.

The nitrogen cycle is complete when the water readings are as follows:

  0 to 2 weeks 2 to 4 weeks 5 to 6 weeks Complete
Ammonia  <0.06 ppm  0.0 ppm  0.0 ppm  0.0 ppm
Nitrites  0.0 ppm  <0.75 ppm  <0.75 ppm  0.0 ppm
Nitrates  0.0 ppm  0.0 ppm  25.0 ppm  25.0+ ppm


Once the nitrogen cycle has stabilized as indicated by the above readings, you can gradually introduce more aquatic life into the tank gradually. Water tests should still be taken weekly, or whenever the water appears murky or when problems are suspected. Increasing the number of aquatic life will alter the chemical readings temporarily. This illustration is based on adding a few hardy fish to begin the nitrogen cycle. If you add more than a few fish to your aquarium during the initial few weeks, you will endanger your aquatic life and it will take longer to stabilize the nitrogen cycle. It is important that your aquarium is well-oxygenated. Nitrosomonas bacteria need oxygen to develop and grow. Without the oxygen, nitrosomonas cannot populate, allowing toxic ammonia to build up quickly.

nitrogen cycle

New Tank Syndrome:

This is one of the most critical stages in setting up an aquarium and the most common area of failure for beginners. If you do not allow your aquarium to complete the nitrogen cycle before adding aquatic life, it can result in New Tank Syndrome, or ammonia and nitrite poisoning. Common symptoms of New Tank Syndrome in fish include loss of coloring, hiding in corners with clamped fins and lying near the bottom of the aquarium. Ultimately, New Tank Syndrome is lethal to your aquatic life.

Resources and recommended supplies:

To learn more download our Aquatic Success Kit.