How to Cycle a New Aquarium

Setting up a new aquarium without losing your fish.

Whether freshwater or marine, all new aquariums must go through a period of "cycling".

Sometimes referred to as the nitrification process, the biological cycle, the nitrogen cycle, or new tank syndrome, cycling a tank involves establishing enough beneficial bacteria to break down the toxins the fish naturally create. Over time, this beneficial bacteria will colonize the aquarium's filter and gravel bed. Trying to rush this process usually results in dead fish and frustrated aquarium owners.

The process of cycling begins when the first fish are added to a new aquarium. When the fish are introduced, a small number of bacteria are introduced with them. Ammonia is emitted into the tank as a result of fish waste and uneaten food. Bacteria called nitrosomas consume ammonia which is very toxic to fish, and break it down into nitrite.

The resulting nitrite, which is also highly toxic for fish, is then consumed by bacteria called nitrobacters, which convert the toxic nitrites into nitrate. Nitrate is not nearly as harmful to fish as nitrites or ammonia, but it is dangerous in high amounts. By performing regular partial water changes, aquarium owners prevent nitrate from increasing to stressful levels.

Live aquatic plants will use up some of the nitrates in a freshwater aquarium. For marine (or saltwater) aquariums deep sand beds or live rock can be home for anaerobic denitrifying bacteria which breakdown nitrites into harmless nitrogen gas.

You can start the cycling process of your new aquarium by introducing a small number of hearty fish. Be sure to use fish that you will want to leave in your established tank. Do not use cheap feeder goldfish, unless you intend to keep goldfish in your tank. For tropical community tanks, a small school of zebra danios, or white clouds work well.

For a more aggressive tropical tank, tiger barbs are a good choice. Usually two to four fish are a sufficient number to start the cycling process. Adding too many fish will produce more waste than the small amount of bacteria can sufficiently break down, and will result in stressed fish and higher death rates.

Be careful to not overfeed the new fish during this process. Excess food falls to the bottom and begins to decompose, adding more ammonia to the aquarium. A small amount that the fish can eat in about 3 minutes is sufficient. Be sure to remove uneaten food by skimming it off the top with a fish net or sucking it up with a gravel vacuum.

After adding the initial fish, partial water changes of 10-15% should be performed every couple of days to remove excess nitrates. During this time, keep an eye on the ammonia and nitrite levels in the aquarium, either by using an aquarium test kit, or having your local pet store test the water for you.

Most pet stores will check aquarium water quality for free. The ammonia and nitrite levels should rise and then gradually drop off to safe levels. Be patient. This process usually takes 3-4 weeks, but can take as long as 8 weeks.

After the ammonia and nitrite levels drop back to safe levels, more fish can be added to the aquarium. Be sure to choose fish that are compatible with the fish you used to cycle the tank. Add new fish slowly, introducing small numbers at a time so as not to overwhelm the newly established bacteria filtration. If you add too many fish at one time, it may be more than the established bacteria can break down and the ammonia and nitrite levels could spike again causing dangerous results for the fish.

The key to establishing a healthy new aquarium is patience. Follow these guidelines, resisting the temptation to fill up the aquarium with fish too quickly, and you will be well on your way to owning a beautiful and enjoyable aquarium. It will be worth the wait.

Speeding up the Cycling Process

Feeling impatient?

Here are a few things you can do to speed up the process, although nothing works as well as being patient and letting time do the work

  • Borrow some gravel from an established aquarium. If you have a friend with an established tank with healthy fish, It should already be loaded with the beneficial bacteria your aquarium needs
  • Try used filter media. A used filter cartridge or other filter media from an established and healthy tank also contains beneficial bacteria. This gives the colonizing bacteria a good jumpstart and speeds up the cycling process.
  • Raise the temperature of the aquarium to between 80 and 82 degrees Fahrenheit.
  • There are a number of products on the market that claim to contain beneficial bacteria. Ask for help in finding these products at your local pet store.