How to Start a Tropical Fish Tank Cycle

New tropical fish tanks require bacteria to create a continuous biological cycle that provides a healthy environment for the fish. This is known as the “Nitrogen Cycle” and can take from two to eight weeks (or more) to complete. This cycle begins with ammonia released from fish waste and uneaten food, which forms a bacteria. The bacteria convert the ammonia to nitrite, but both of these can be toxic to fish. However, they will not typically prove deadly to the fish, as long as you take the necessary steps. Through time, another “good” bacteria (nitrobacter) is formed and the cycle is complete.

Set the aquarium in the desired location. Add gravel and decorations. Put the heater and filter in place along with any other items, such as air stones. Fill the tank.

Wait one hour to allow the heater to stabilise and then turn everything on. Check to make sure all equipment is working.

Wait three days for the temperature in the tank to stabilise. Obtain two or three hardy fish (per 10 to 20 gallons of water) to start the fish tank cycle. Some fish to consider, according to Animal World and Fish Lore, include zebra danio, white clouds, cherry or tiger barbs and pseudotropheus zebra. Choose fish you plan to keep in the tank, not feeder fish to use and then discard, as some used to suggest years ago.

Feed the fish sparingly, since you do not want to create too much ammonia at one time. This could be harmful to the fish. Change 10 to 15 per cent of the water three times the first week. Changing the water removes any high amounts of toxins and prevents the fish from being harmed. Test the water as directed with the aquarium tester kit (listed in the “Things Needed” as optional). Or take it to your local pet store for testing, if you do not want to purchase a water test kit. These kits test the ammonia, nitrites, nitrates and pH levels. The ammonia in the tank will most likely be high at this time, which is typical.

Continue to test the water every week or two. Monitor the levels of ammonia, nitrite and nitrate, which will fluctuate as the tropical aquarium goes through the nitrogen cycle (for up to two months). The nitrite level will rise and the ammonia level will come down. Eventually the nitrite will come down, as the ammonia did.

Add a few new fish, once the ammonia and nitrite levels are zero and nitrate is present in the tank. Do not add more than a couple new fish at a time to allow the filter to handle the additional waste.


Rinse anything you plan to put into an aquarium thoroughly with water. This includes even gravel, filters and decorative items.

Things You'll Need

  • Fish tank
  • Gravel
  • Decorations
  • Water
  • Heater
  • Filter system
  • Fish
  • Fish food
  • Water test kit (optional)
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About the Author

Diane Dilov-Schultheis has been writing professionally since 2000. She is a food and travel writer who also specializes in gaming, satellites, RV repair, gardening, finances and electronics. She is a member of the Society of Professional Journalists and has been published online at the Travel Channel and Intel.