This article is based on my own experience as well as tons of reading I have done on the subject, hope you find it useful!
Most people who leave the fishkeeping hobby do so because they become frustrated when all the fish die. They go buy more and they die. Most of these fishy deaths can be avoided with a basic understanding of the nitrogen cycle and close monitoring of your water. So arm yourself with a test kit, and let's get started.
Fish release ammonia through respiration and bodily secretions. Ammonia is also released when fish wastes, excess food, or dead fish, are allowed to decay in the aquarium. This is a good reason to be cautious not to overfeed and to maintain a gravel cleaning regimen. Ammonia is toxic to fish and if allowed to rise too high, can quickly bring about fish deaths. In a new aquarium, this will rise rather quickly because the nifty little bacteria called nitrosomas aren't established yet. They break the ammonia down into nitrite, which is not quite so toxic as ammonia. Nitrosomas reproduce by splitting in half about every 7-8 hours. Imagine how long it would take to reach a number of a few thousand bacteria if you only start with one... Nitrosomas will adhere to just about anything they can hold on to such as gravel, inside of the filter, filter sponges, etc. Within the first couple weeks, you should see a decrease in the ammonia levels, and the nitrite levels will begin to rise.
Next to begin colonizing are the nitrobacter. They use the nitrite present in the tank from the ammonia that has been broken down. They begin growing after the nitrosomas are established and the ammonia level has dropped, doubling about every 13 hours. Nitrobacter will adhere to the same type of surfaces as the nitrosomas. Nitrobacter will produce nitrate from the breaking down of the nitrite. Nitrite can be harmful to your fish as well if allowed to exceed a safe level. When you begin to see your nitrite drop, you can then expect a rise in nitrates. In a fairly new aquarium, you may see a slight spike of ammonia and therefore nitrite if you add a significant number of new fish. A fully cycled, mature aquarium should have no detectable ammonia or nitrite.
If it does, you need to determine why they are present and resolve the problem as quickly as possible.
Nitrates aren't nearly so toxic as it's predecessors. Most fish can tolerate higher levels of nitrates if they are in the tank as the level slowly rises, which probably explains how some people claim they haven't done a water change in a year! and have only replaced what has evaporated out. The problem with that theory is that growth is inhibited by high levels of nitrates--fish that live in this sort of environment will generally not live as long and not reach their full growth potential. Another problem with high nitrates comes when trying to add new fish. Although the current occupants may have adjusted to the best of their ability, they have had a long time to do so. Often the pollutants in the water are too numerous for a new fish accustomed to clean water to adjust to, and they frequently become ill and die. High nitrates are also said to lower your fish's resistance to disease. As a general rule, nitrates below 25-50 ppm
shouldn't harm your fish or stunt their growth. Overall, for most people the easiest method to reduce nitrates is regular water changes. Your fish will love you for it!
Nitrosomas and nitrobacter are aerobic bacteria, which means they require oxygen to survive. Their numbers will be affected by lack of oxygenation, such as gravel that has excess waste and has therefore formed anaerobic pockets (without oxygen), or in areas of the tank which don't receive adequate oxygenation from water flow. When cleaning your tank, be careful not to clean too well
. Don't clean your filter, inserts, gravel, etc. all at once--you may risk killing off a significant portion of your beneficial bacteria. Chlorine and chloramines will kill them, as well as hot water, so take care when rinsing filters and such. Both nitrosomas and nitrobacter require micronutrients that are not found in reverse osmosis, distilled, or other such forms of purified water. Some antibiotics will kill off your beneficial bacteria as well, as they were designed to kill bacteria! When treating with antibiotics, you should check to see that you don't have an increase in your pollution regardless of what the local fish store tells you!
How much of each particular pollutant is too much will depend on your individual fish and each's tolerance. Try to start with a hardy type of fish, and keep your levels of ammonia of nitrite at the low end of the scale that comes with your test kit. Remember in a new aquarium, you don't want to remove all of the ammonia and nitrites because the bacteria need food to thrive. When you notice levels climbing too high, do a partial water change and recheck your water to see how much you have removed. You may have to do small, frequent water changes at first to keep things optimal. Good luck!