How to Cycle a Tank Using Old Water

One of the most important steps in setting up a new aquarium is starting the nitrogen cycle. Fish produce waste in the form of toxic ammonia, which is converted by helpful bacteria into nitrite, and finally nitrate. Most new aquarium owners use inexpensive fish to begin the cycle, but this can take several weeks for the cycle to establish, and can be fatal to the fish. Another way to start the process is to add water from an existing, disease- and pest-free aquarium.

Things You'll Need

  • Dechlorinator
  • Small container
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Instructions

    • 1

      Fill the tank with fresh water and add a dechlorinator to remove chlorine if using chlorinated tap water. Allow the water to reach room temperature.

    • 2

      Scoop a small amount of water from the existing tank using a clean container.

    • 3

      Pour the old water into the new tank to seed the tank with bacteria. This will start the cycling process.