How to Know When an Aquarium Has Cycled

Cycling an aquarium is the process of removing ammonia and nitrites from the tank, while growing good, nitrifying bacteria so that live fish can be placed in the water. The good bacteria helps eat the ammonia and nitrites left from fish going to the bathroom in the tank. An aquarium that hasn't been cycled is more likely to host an unsanitary environment where fish are sickly and may die. Cycling allows you to add fish to the tank without fear of ammonia or nitrite poisoning. Use these tests to determine if your aquarium has finished cycling.

Things You'll Need

  • Ammonia test kit
  • Nitrite test kit
  • Nitrate test kit
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Instructions

    • 1

      Test the water for ammonia levels a few days after you've introduced nitrifying bacteria to the tank. Ammonia levels must be at zero before you can add fish to your aquarium. Ammonia levels will spike then quickly go to zero when the tank finishes cycling. To perform the ammonia test, either drop a chemical solution provided with the test into a tube of aquarium water and compare its color to a color chart on the bottle, or dip a test strip into the aquarium and compare its color to a color chart on the bottle. Most aquarists prefer the test strip method for its ease of use.

    • 2

      Test the water for nitrite levels. Nitrite tests come in strip form and are performed the same way as ammonia tests -- you dip a test strip into the tank and compare its color to a color chart on the bottle. Nitrite levels will spike a few days after adding nitrifying bacteria, but they must be at zero before you can safely add fish to your aquarium. Nitrite poisoning is one of the biggest fish killers. Nitrites are more poisonous to fish than ammonia, so it is essential that nitrites are gone before you add fish to your aquarium.

    • 3

      Test the water for nitrates. Nitrate tests come in strip form as well, and work the same as the other tests in that you dip the test strip into the tank and compare it to a color chart on the bottle. Nitrates are a natural by-product when the good bacteria eat the ammonia and nitrites. Nitrate levels must be less than 10 ppm, or parts per million, before you can add fish to your aquarium.

    • 4

      Change 50 percent of the water at a time to bring nitrate levels down quickly. Cycling sometimes causes nitrate levels to spike if you add a large amount of ammonia to jump-start the bacteria. It's all right to change 50 percent of the water because the good bacteria is still in the filter and on the tank walls and rocks.