Cycling is essentially waiting for the bacteria that process the ammonia in fish waste to grow into sufficient numbers to do their job. "Cycle" refers to the Nitrogen Cycle, where ammonia, NH4 is turned into nitrites, NO2 and then into nitrates, NO3.
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. Yoru should notice these milestones:
Basically, once nitrates appear, you are there. Just make double sure ammonia and nitrites are zero before adding more fish.
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.
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 economical than dip strips in the long run, and many find them easier to read.
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.
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