What Causes High Levels of Ammonia in Freshwater Fish Tanks?

The main cause of high ammonia in an aquarium is the presence of fish. When you introduce fish and start feeding them, you need to take measures to keep ammonia levels low. High ammonia levels are lethal to fish, but even slightly elevated ammonia is bad for them. Nonlethal ammonia build-up increases susceptibility to disease, shortens lifespan and decreases coloration.

  1. Overcrowding and Overfeeding

    • Ammonia builds up in the water of the fish tank when nitrogen-containing substances decay. The two main nitrogen sources in an freshwater fish tank are the waste excretions from the fish and uneaten food. One of the breakdown products of both these substances is ammonia. In addition, excessive amounts of uneaten food lead to unpleasant, cloudy water.

    Lack of Beneficial Bacteria

    • Certain bacteria use ammonia as food. Bacteria of the genus Nitrosomonas eat ammonia and are common in fish tanks. However, these bacteria in turn produce nitrites. A second family of bacteria, genus Nitrobacter, converts nitrites to nitrates. Both nitrites and nitrates are harmful to fish at high levels, but less so than ammonia. If your fish tank lacks either Nitrosomonas or Nitrobacter bacteria, ammonia levels can rise rapidly. Encourage correct bacterial levels by providing good oxygenation, such as with airstones or water filtration, or by innoculating your tank with bacteria from a pet store. You can also get beneficial bacteria into your tank by transferring in a small amount of gravel from a fully established, disease-free aquarium.

    Not Enough Plants

    • Although nitrates are less deadly than ammonia, high nitrate levels will also hurt your fish. Plants, however, can use nitrates as fertilizer. Aquatic plants in your freshwater aquarium not only take in nitrates, but also recycle carbon dioxide given off by the fish back into oxygen. This benefits both the fish and the bacteria in the tank.

    Infrequent Water Changes

    • Another key tool in eliminating nitrates and ammonia is to remove some of the water and replace it. According to Gregory Skomal, author of "Freshwater Aquarium: Your Happy Healthy Pet," use weekly water swaps of 10 to 20 percent of the tank's volume. For a small 5-gallon tank, which measures approximately 15.5-by-10.5-by-14.5 inches, a 10 percent water change would amount to swapping out half a gallon or eight cups. Water changes also remove fish waste and uneaten food, and they replace minerals used by the fish, plants and bacteria. Many pet stores carry specialized siphons to make water changes easier, such as hand-pump siphons or extra-long siphons designed to drain into a sink.