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Cycling is waiting for the natural bacteria that process fish waste to grow into sufficient numbers in an aquarium tank to do their job. Without enough bacteria, waste builds up quickly to harm or kill your fish.

"Cycle" refers to the Nitrogen Cycle, where ammonia, NH4, the primary poison in fish waste, is turned into nitrites, NO2 (better, but still damaging), then the nitrites are turned then into nitrates, NO3(harmless, except in extreme concentrations.)

Bad News, Good News. The bad news is ammonia. It is part of fish waste, and is harmful to fish. While you can remove the goopy part of fish waste with gravel vacuums and filter rinses, ammonia disperses in the water and cannot be cleaned out directly. The good news is that certain bacteria naturally break ammonia down into less harmful forms. These bacteria colonies will appear on their own in your aquarium (as long as there is ammonia for them to eat). The bad news is it takes time for it to grow to a large enough colony to keep up with the job.

What Happens When Cycling Happens. In a new aquarium, the cycle begins when ammonia is excreted in fish waste. Bacteria begins to grow in the filter, gravel and other surfaces of the tank. After a week or two, bacteria that breaks down ammonia into nitrite (NO2) grows to sufficient numbers to begin to get ahead. Ammonia levels decrease, nitrite appears and increases. After a two or three weeks, bacteria that breaks nitrites into nitrates grows to sufficient numbers to get ahead. Nitrite levels decrease, nitrate appears and increases. After after three or four weeks, all the bacteria players are in place. All ammonia is virtually immediately processed into nitrites, and the nitrites in turn processed into nitrates so quickly that ammonia and nitrites are detectable only in trace amounts if at all. Nitrates are removed by water changes. If fish are removed, ammonia is no longer being added, and the cycle bacteria will eventually starve and disappear.

Not Fit For Man Nor Fish. Until the bacteria colonies are in place, your aquarium is not safe for sensitive fish or large numbers of tolerant fish. Ammonia and nitrites are harmful to fish. Nitrates are not harmful, except in large concentrations--which are avoided by weekly water changes.

Stop Cleaning Already. If the tank is cleaned too well (e.g., gravel and ornaments are washed, tank walls scrubbed and filter media replaced at the same time), too many bacteria may be removed and the cycle will have to start over. Or, ammonia and nitrites may persist until the bacteria catches up (a "mini-cycle"), if you managed to leave some bacteria intact. Clean only a few elements at a time, cleaning items in turn week to week rather than all at once.

Change is Good. Nitrates are generally harmless. But if the tank water is never changed, nitrates and other dissolved solids can increase to unhealthy levels, even though the fish look fine. Unchanged water may not appear to harm the fish that have been living in it for a long time because they have adapted to it slowly as the levels gradullay increased. But if you suddenly change the water, or move the fish to a new tank, those old fish may suddenly die from the shock of the changed water chemistry. New fish added to the unchanged tank may die for the same reason. This state of affairs is called "Old Tank Syndrome." Thus, it is best to have a regular water change schedule.

More Changes are Better. Topping off a tank that has lost water from evaporation does not count as a change--the nitrates never leave. Every week or two, you should remove water and add some back. The amount varies with the kind and number of fish. The more fish, the more messy the fish, and the more sensitive the fish, the more water you should change more often. Typical amounts are at least 10 to 30 percent a week.

How to Cycle Using Fish. Start with a hardy, inexpensive fish. Zebra Danios are a good choice for a tropical tank--and they are interesting fish to keep. Start with only two or three fish for most tanks 55 gallons and under. Feed them very sparingly, only once a day. You want a steady but small supply of ammonia. This will get the bacteria started without spiking ammonia to dangerous levels for the fish. Test ammonia, nitrites and nitrates every day. You should notice these milestones:

  1. Ammonia levels rise, nitrites and nitrates not above tap water levels.
  2. Nitrites appear.
  3. Nitrites rise, ammonia levels begin to decrease
  4. Nitrates appear.
  5. Nitrate levels rise, nitrites begin to decrease, ammonia almost gone.
  6. Nitrites and ammonia near zero, nitrate levels continue to rise.

Basically, once nitrates appear, you are there. Just make double sure ammonia and nitrites are zero before adding more fish. But the bacteria colony is still small; only add a few new fish at a time, then wait a week or two, to give the bacteria colony time to catch up.

Ammonia Emergency Procedures

What if things have already gone wrong and ammonia is spiking?

  1. Stop feeding. Feeding means fish waste which means ammonia. Stop feeding until ammonia levels go down.
  2. Change the water, up to 90%.Remember to remove chlorine and match temperatures! Then change the water, again up to 90%, every day or more, testing to make sure ammonia levels stay low. These water changes will not remove the bacteria you are trying to encourage; they are living on the surfaces, not floating free in the water.
  3. You can also purchase an ammonia-locking liquid at the pet store as a stop-gap. That does not replace the need to change the water aggressively. Achieving ammonia levels through proper cycling procedure is always superior to adding chemicals.

Cycling Alternatives

Cycling can be hard on fish. Ammonia and nitrates can damage their gills. Only certain fish are considered hardy enough to withstand cycling. But be prepared to have a place for them once cycling is complete. If you do not want to cycle with fish, there are several alternatives.

  • Filter Media or Gravel from Another Tank. If you have another active tank or a friend with the same, place new filter media for your new tank in the established tank for a week or two. Bacteria colonies will grow in it. Or, grab a handful of gravel from the established tank and put it in a mesh bag. Move it to the new tank, and add a few fish right away. The bacteria colonies will be ready to go. Only add a few fish--too many will overwhelm the new colonies. This is the easiest method, and a great way to quickly build a quarantine tank when needed for new or sick fish.
  • Fishless Cycling is the process of adding pure ammonia straight to the tank, with no fish present. Only use plain, unscented liquid household ammonia. For details on the procedure, see this page. Similar results can be achieved by adding fish food.
  • "Instant" Cycling Products. These, such as Tetra SafeStart, are essentially bacteria colonies in a can. Just add a can full to your new tank along with your new fish. Follow the directions very carefully. . With this method, you will be adding most of your fish at the same time as the product. (The other cycling methods require that you start with only a few fish to avoid overwhelming the bacteria colonies.)

Test Time. No matter what method you use, you will need water test kits to monitor the process so that you will know when the cycle is complete. We recommend kits for ammonia, nitrites and nitrates. When both ammonia and nitrites are zero, and nitrates are starting to rise, you know the cycle is complete. (You need all three tests because under certain conditions, you may have all three compounds present. It is not safe to rely just on the nitrate test, for example.) This also gives you something useful to do while your patiently wait for the tank to cycle! Liquid Test Kits are more accurate, easier to read, and more economical than dip strips in the long run.

Not So Fast! Once the cycle is established with any method, add new fish in small numbers. Adding too many new fish at one time to a cycled tank can also get ahead of the bacteria and give you a mini-cycle.

No Way Around It, Gotta Go Through It. Remember that the bacteria need ammonia to get started. Without it, they will never grow in your tank. Just leaving the tank sit with water in it will not prepare the tank for fish. You must provide ammonia (and introduce the bacteria if not using fish to cycle) to get sufficient bacteria colonies to process the fish waste and have a healthy, balanced aquarium.

Small Tanks, Big Trouble Small tanks are more sensitive to cycling problems. The smaller the quantity of water, the quicker the ammonia concentration rises to harm fish. A ten, or better yet, a twenty gallon tank is really no more work to set up and maintain than a five gallon. The larger tank will cycle just as quickly, and the ammonia concentrations will not rise to harm the fish as quickly--assuming you start with just a couple fish and add slowly no matter how big the tank.

How many total fish can your put in your tank? Check our How Many Fish? Calculator for information on how many fish your aquarium tank can support.