Introducing just a couple of hardy fish and then waiting is the time-tested way to culture beneficial bacteria in a new fish tank. These first fish you introduce to the tank will produce ammonia, which beneficial bacteria will eat, causing the bacteria to grow and multiply. Leave the starter fish in the tank and don't introduce any more inhabitants until the beneficial bacteria have become established and converted all of the ammonia to safer compounds.
Basics of the Nitrification Cycle
In the wild, the waste from fish and other aquatic animals breaks down, yielding highly toxic ammonia. Because natural lakes and rivers have low fish densities, ammonia levels also remain low. Aquariums, by contrast, have much higher fish densities than natural habitats do, and ammonia levels can become lethal quickly. When ammonia is present in the water, several types of bacteria, notably Nitrosomonas species, begin converting it to a less harmful chemical, called nitrite. At this point, other bacteria, including several Nitrobacter species, convert the nitrites into nitrates, which are even safer for the fish. By establishing a colony in your aquarium's filter, you can achieve biological filtration with these beneficial bacteria.
Fortunately, the bacteria necessary to initiate the nitrification cycle are ubiquitous -- they are found in the air, on your fish tank's surfaces and, once ammonia is present in the aquarium, in the tank water. However, you must introduce ammonia into the tank first. The easiest way to do this is by introducing a couple of fish into the aquarium. As the fish eat and digest their food, then expel the waste, the waste will raise the water's ammonia levels, providing the bacteria with a food source. It is important to avoid overfeeding your starter fish, as this can cause the ammonia levels to rise too quickly.
The best fish to use for cycling purposes are hardy, inexpensive and comfortable in the same water conditions as the tank's intended long-term inhabitants are. For example, goldfish are excellent for cycling cold-water tanks, while zebra danios, guppies, pupfish and barbs work well for jump-starting warm-water tanks. Damselfish are effective for cycling saltwater aquariums.
Testing and Retesting
Once you add your starter fish to the tank, begin testing the water frequently. Initially, ammonia levels in the tank will rise as the fish excrete waste. A few days or weeks later, the ammonia levels will drop precipitously, while nitrite levels will increase dramatically as they begin feeding on the ammonia. A short time after this, the nitrite levels will plummet and nitrate levels will rise. Once the nitrite and ammonia levels reach zero and nitrates are present, your tank water has completed the cycling process, and it is ready for more fish. It may take up to six weeks to complete the cycle; if the tank water is below 70 degrees Fahrenheit, it may take even longer.
The Fishless Approach
Some aquarists prefer to add general-purpose ammonia to their tanks to initiate bacterial development. This method requires extensive experimentation to get the ammonia levels correct, as different commercial ammonia products are mixed at different dilutions. Begin by adding a very small amount of ammonia to the tank. Wait several hours for the tank water to mix thoroughly, then test the water. Strive to attain ammonia levels of 4 parts to 5 parts per million.