Exotic Animal

Understanding the Nitrogen Cycle in Your Fish Tank

May 18 • 3 minute read

Are you new to the hobby of fishkeeping? You may or may not have heard of something called the “nitrogen cycle.”  This may sound like a complicated scientific concept, but it’s important to understand how this process works because the health and well-being of your fish depend on it. The nitrogen cycle is the foundation on which your new aquarium is built.

The Steps of the Nitrogen Cycle

When aquarium hobbyists talk about the nitrogen cycle, they’re referring to a chain of biological reactions that occur when a fish is introduced to an aquarium. It begins when decaying food and fish waste turn into toxic nitrogen compounds, including ammonia, nitrites, and nitrates. In high concentrations, these compounds can sicken or kill your aquarium fish.

For the nitrogen cycle to work effectively, you must have plenty of beneficial bacteria and plants to consume and convert waste products. It takes up to three months for a new fish tank to process waste and convert it into nitrate. That said, ammonia and nitrite levels should be zero for your tank to be fully cycled.

Here are the stages of the aquarium nitrogen cycle:

Stage 1: Create Ammonia

Ammonia forms from the metabolism of fish urea and proteins. Ammonia from fish is primarily excreted from the blood through the gills via passive diffusion. Low quantities of ammonia are lost through feces and urine. Ammonia is also introduced into the fish tank when waste breaks down. These include dead pieces of plants, uneaten fish food, decaying algae, dirty filters, and other decomposing organic matter.

The concentration of ammonia in a fish tank could get too high if there are too many fish in the tank or if the fish are being given more food than they need. High concentrations could cause stress, internal damage, or death. But in a well-balanced aquarium environment, nitrogen-fixing bacteria (Nitrosomonas) consume ammonia, converting it into nitrite.

Stage 2: Convert Ammonia to Nitrite

Nitrite is toxic and the most common killer of aquarium fish, so you need to be extra careful during this stage of the nitrogen cycle. Nitrite levels usually build up by the end of the first week or during the second week after introducing fish. This compound is toxic to fish because it binds with the hemoglobin in the fish’s blood and converts it into a compound called methemoglobin. This can lead to suffocation because methemoglobin prevents red blood cells from transporting oxygen through the body.

To prevent high levels of nitrite in your fish tank, feed sparingly and make sure there are not too many fish in the tank. Also, carry out a regular, partial water change. Use well-aged water instead of tap water. Nitrite levels should then be tested once per week. Your fish tank should have no detectable levels of nitrite after the cycling period.

Stage 3: Convert Nitrite to Nitrate

Nitrates are produced when another form of nitrifying bacteria (nitrobacter) breakdown nitrites. Except in very high concentrations, nitrate is relatively harmless to fish. The biological food chain cancels out the toxic effects of nitrite and ammonia.

To prevent too high a buildup of nitrates, you’ll need to filter your fish tank water and change a portion of it regularly. Maintaining a small animal population and feeding as needed is also important. If you have aquatic plants in your aquarium, they can help remove and use nitrates.

Stage 4: Maintain Nitrate Levels

A major part of healthy fish keeping comes down to maintaining the delicate balance in the nitrogen cycle. When the water has been rid of toxic nitrite and ammonia, you need to keep nitrate levels at a level that fish can tolerate. This involves:

  • Regular water changes
  • Using quality carbon filters
  • Overfeeding prevention
  • Cleaning sand bed
  • Maintaining a small animal population
  • Gradually introducing any new fish
  • Having enough plants

The Foundation of a Healthy Fish Tank

The nitrogen cycle is one of the important steps when setting up a new fish tank and shouldn’t be overlooked. When you have a solid understanding of biological filtration, your fish are more likely to be healthy and happy.

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