The following is a basic guide on cycling and a more advanced explaination is in the works.
Cycling is preparing a tank for fish to live in, people new to the hobby will often dive head first into overstocking their new tank and then wonder why fish start dying off after a week or so.
Basicly a fish gives off Ammonia (NH3,NH4)from fecal matter which is broken down by nitrifying bacteria into Nitrite (NO2) which in turn is broken down to Nitrate(NO3).
Ammonia and Nitrite can be harmfull and more often enough lethal to fish at any level and in a fully cycled tank both should read 0. Nitrate is not as lethal and “some” fish can live in high levels but its best to keep it under 20ppm(parts per million) once the tank is fully cycled.
Once the tank is fully cycled the nitrates can be kept to a safe level with regular weekly water changes.
1. Obtaining gravel and/or water from a established tank. I would advise against using gravel from a pet shop tank due to risk of disease, parisites etc.
2. Using a filter with media which has been running in an established tank.
3. Plant the tank with potted plants (this really helps)
4. Using products sold in aquarium stores like wardlys cycle.
5. Using break in fish like roseys. Theses fish should be healthy on purchase and only 2 or 3 will be needed for the break in service.
Ok the following is only a guide and only by testing with a kit that does Ammonia,Nitrite and Nitrate you will know the true state of the tanks progress.
Start by doing “no” water changes for about 2 weeks by which time the Ammonia level should be comming down with nitrite levels spiking. Then do about a 20% water change. Then after about another 2 weeks of no water changes the nitrite should be declining with nitrates starting to rise. If all goes well you should be able to start regular 20 to 30% water changes. (During the cycle feed the fish sparingly to avoid a bio overload effect) A fully cycled tank will read Ammonia 0,Nitrite 0,nitrate under 40ppm and under 20ppm or lower is best. If your using fish that you want to keep and they start showing signs of severe stress you can do 5 to 10% weekly water changes (More in real bad cases) This will naturally lengthen the time it takes to cycle the tank considerably.Also adding aquarium salt (or non iodized cooking salt) at a rate of 1 table spoon per 5 US gallons (about 20litres) of water will help will help reduce the effects of nitrite toxicity.
The actual time it takes can vary a lot as temperature,ph,whether or not you use gravel,filter media from an established tank and other factors can play an effect on how fast or slow a cycle will finish.
Also note that a tank doesnt have to be new to restart a cycle. This can be caused by too much off a water change too often, overloading a tank with too many fish, using certain medications will kill off the nitrifying bacteria needed to keep the tank healthy.
There is another method of cycling called a Fishless Cycle which involves using pure ammonia products commmonly sold in hardwares and supermarkets in the cleaning sections. I personally am yet to try it but have heard pretty good things about it.