Nitrogen Levels in an Aquarium

Nitrogen is one of the most common elements in nature. In a fish tank, nitrogen compounds go through a cycle from ammonia to nitrite to nitrates. Nitrates building up in a tank can be stressful to the fish and lead to algal blooms. Maintaining proper nitrate levels requires consistent monitoring and establishing bacteria colonies that will lower nitrates level in the tank.

  1. The Nitrogen Cycle

    • The nitrogen cycle occurs in three stages. The first stage is the production of ammonia or ammonium. These compounds enter the tank through waste produced by fish or by uneaten food. Ammonia is harmful to fish and will appear when pH is over 7. Ammonium is not harmful to fish and will occur at pH levels of 7 and below. The second state of the cycle is when nitrosomonas bacteria develop in the tank and breaks the ammonia or ammonium down into nitrite (NO2). The third and final stage occurs when the nitrite is converted to nitrate (NO3) by nitrobacter bacteria.

    Nitrates

    • Nitrates are less harmful to fish than either ammonia or nitrites present in the tank. The main issue is nitrates can build up to harmful levels in time causing stress on the fish. To rid the tank of nitrates requires partial water changes. In freshwater tanks plants will help remove some of the nitrates from the tank. In saltwater tanks live rock and deep sand beds will create anaerobic areas where de-nitrifying bacteria can break down the nitrates into nitrogen gas that leaves the tank naturally.

    Levels of Nitrates

    • In most cases when testing for ammonia or nitrates within the tank, you get a reading on the testing strips indicating that the levels are already too high. For nitrates specifically, most fish can survive in tanks with nitrate levels of 10 to 40 parts per million. Extreme stress levels on fish is reached at 60 parts per million. Most plant life present in the tank will die off prior to reaching 60 ppm. Saltwater tanks with invertebrates such as sea anemones are much more sensitive to nitrate levels with the suggested level being .225 ppm or less.

    Starting the Cycle

    • Establishing the nitrogen cycle and getting a healthy level of nitrates within a new tank can take anywhere from two weeks to months. It is not suggested to start a new tank with lots of fish, because the risk of die-off is large. Some hardy fish such as zebra danio in freshwater tanks or damsel fish in saltwater tanks can survive the initial cycle fluctuations. Methods to start the cycle include getting bacteria from existing tanks, placing food in a fishless tank to decompose to start the cycle and placing store-purchased bacteria into the tank.