How to Cycle an Aquarium With Biofilters

When setting up a new fish tank, you must give the aquarium enough time to run through the nitrogen cycle (the breakdown of ammonia to nitrites and to nitrates) and establish a biofilter. This is called cycling the tank. A biofilter is a natural system made of bacteria that turns fish waste, such as ammonia, into less harmful substances, such as nitrates. It takes patience to establish this cycle, but adding too many fish too soon will likely lead to fish death.

Things You'll Need

  • Fish tank set up
  • Aquarium water test kit
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Instructions

    • 1

      Set up your fish tank. If you need assistance with this, consult the fish professionals at the store where you purchased the tank. Fill the tank with water, and let sit for 2 to 3 days. This allows chlorine and other harmful chemicals to evaporate from the water.

    • 2

      Add just 2 or 3 hardy fish to your tank. Good tropical species that are tough enough to withstand the high levels of nitrites and ammonia during tank establishment are guppies, platies and zebra danios.

    • 3

      Test your tank water daily during the first 2 weeks. Ammonia levels will rapidly increase, stressing the fish. You can change 15 to 20% of the water every few days to prevent fish death. During this time, colonies of bacteria capable of converting ammonia to nitrite are growing. These colonies will be large enough to reduce dangerous ammonia levels to near zero within 1 to 2 weeks.

    • 4

      Test your tank water for nitrites starting at week 2. Nitrites will peak at around 1 month, at which time colonies of bacteria capable of converting nitrite to nitrate are large enough to start lowering the nitrite level. Continue to do occasional 15 to 20% water changes if your fish show signs of stress such as gasping at the surface, shimmying, or fins tightly clamped to the body.

    • 5

      Once nitrite levels have dropped, your biofilter is established, and your tank is cycled. You can now add more fish. A good rule of thumb is that an aquarium can only support around 1 inch of fish per 1 gallon of water. For example, if you have a 20 gallon tank, only add the amount of fish that would add up to 20 inches total.

    • 6

      Continue to test your water weekly for nitrates. If you are changing 15 to 20% of your water each week, the nitrate levels should remain in the safe zone of below 20 ppm. If the nitrate level goes higher, change 25% of your water and be careful not to overfeed fish or overcrowd the tank.