What is the nitrogen cycle?
The nitrogen cycle is the most important thing to understand when setting up and keeping an aquarium. This natural cycle is the process by which helpful bacteria break down organic waste into less harmful compounds. It starts with fish waste, dead plant parts and any other organic materials being broken down into ammonia. This ammonia is converted into nitrite by nitrosomonas bacteria. Both of these substances are poisonous to aquatic life and can be dangerous even in low levels. After nitrite is produced, further nitrobacter bacteria break down the nitrites into nitrates. Nitrate can then be removed by water changes and some will be used by plants and algae. All of these bacteria are naturally present in the aquarium from the beginning, but it takes time for them to grow to a level necessary for them to efficiently process the waste in the tank.
Why is this important?
Without a properly established nitrogen cycle in an aquarium levels of ammonia and nitrite will quickly rise to levels that are toxic to your fish. This will lead to fish diying or becoming sick.
How long does this take?
This all depends on how the tank was setup and what kind of tank it is. In a average freshwater tank 6-8 weeks is normal. In a marine tank this can vary but anywhere between 2-10 weeks is normal. Now with these time lines this does not mean the tank is fully cycled it just means that there are enough bacteria to handle small amounts of fish waste in the tank and fish should be added slowly. You know the cycle is done when ammonia and nitrite test at 0 and there are readable amounts of nitrate.
How do I get the cycle started?
Again, this depends on the tank. With freshwater tanks adding a few small hardy fish can add just enough waste to get the cycle started. This is a little risky for those fish though since they will be exposed to ammonia and nitrite which could harm them. The best way to get the cycle started without adding fish is by simply adding some fish food. As the food breaks down ammonia is created which starts the nitrogen cycle. In marine tanks the best method is by adding live rock and feeding the tank, since live rock and live sand already contain bacteria the process has a jump start without needing to add live fish. Unfortunately the delicacy of many marine fish means it is still advisable to give the aquarium an appropriate period time to completely cycle and stabilize. It is not uncommon to cycle a marine aquarium with hardy fish like damsels, but this has its drawbacks. The primary issue with this is the difficulty in removing damsels from an established tank with live rock if you no longer want them due to the aggression they develop as they mature. In recent years there has been an increase in products that speed up the process of cycling your tank. These live bacteria supplements are advertised as a method of instantly cycling your tank so you can add fish immediately. I personally haven't tried many of these products so I can't say how well they work but I have used live bacteria from existing established tanks. This method uses filter material, gravel or bio material from a tank that has been running and has an established amount of bacteria.
The nitrogen cycle is the most important thing to complete when starting a new tank. The misunderstanding and lack of knowledge of the nitrogen cycle is why tanks fail and people get discouraged in keeping fish. It is a process that takes a little time but it is integral to the longevity and health of your tank, and as we often say "patience is the most important tool for success with an aquarium."