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|The Nitrogen Cycle
If your ph is higher than 6.8..
Fish create ammonia as part of their biological processes. It is also created by decaying material in the tank like uneaten food and dead plants. Ammonia in an aquarium has no place to go but into the water. Ammonia is a highly toxic chemical and only small amounts are needed to be deadly. In your filter, gravel, on plants, the glass, the ornaments...(you get the picture) there are millions of microscopic creatures that have different roles in life. These creatures, known as bacteria establish what is known as the nitrogen cycle. These bacteria are aerobic, which means they need oxygen to do their work. A group of bacteria known as nitrosonomas, break down the ammonia into nitrites.
Nitrites are also lethal in small amounts, but not as small as ammonia. Nitrites are also broken down by bacteria. This group of bacteria, known as nitrobacters, also requires oxygen to do its work. The nitrites get broken down to nitrates.
Nitrates are not lethal except in extremely large amounts. There are only a few ways to regulate the amount of nitrate. Plants remove nitrate by using it as food. There is a type of bacteria (anaerobic) that will consume nitrate, but oxygen drives them away, and you need a large amount of surface area exposed to very slow moving water in order for them to work. Large amounts of this bacteria are lethal to fish. The other way of removing nitrate is through water changes.
If your tank is new...
The ammonia eating bacteria are not yet established. This means ammonia will build much faster than in an established tank. For this reason alone SMALL fish should be added one or two at a time, and only every two to three weeks for the first four months at the least. Each time a fish is added, the ammonia levels need to be monitored daily. When the ammonia begins to drop, both ammonia and nitrites need to be monitored daily until they drop to zero parts per million (ppm). As time goes on, nitrate levels will continue to increase unless it is removed.
Another way to start your tank is the fishless method. By using pure household ammonia and a test kit, you will be able to track the amonia spikes and not kill any fish. Simply add enough ammonia (a few drops at a time), usually 3 drops per gallon, to read 1 part per million then wait for it to decrease then repeat two more times. Put fish in when the ammonia reads zero.
If your tank is established...
The ammonia spikes will still be there but not as high as in a new tank, as long as fish are added sensibly. This is because the bacteria are already established in the aquarium and only need to increase their numbers in order to compensate for the higher amount of ammonia.
If your ph is lower than 6.8..
..and stable at that point, ammonia is not an issue, because at the lower ph ammonia becomes ammonium. Ammonium is harmless except when in very large amounts. It is even easier for the plants to use for food, and is unusable by the nirifying bacteria discussed above.
The Bad Guys...
The bad guys are all of the things that prevent fish from thriving. They come in many forms, such as biological (wastes, hormones, diseases), chemical (chlorine, chloramine, medications), uneducated fishkeepers... as well as many others. Some are actually created by the fish, some will be added by you. All of these things can and must be controlled by you.
If You Still Need to Buy A Tank..
Ok, if you have yet to buy a fish tank or add fish to an existing one, for the sake of your fish, read about them. By reading you will learn their preferences, which fish will get along together, and how big your favorite will get. For example, did you know that a Goldfish will get up to two feet long and live for 70 years IF it is cared for properly. The most important peices of information would be adult size and compatability. You should consider this when buying a tank.