Everywhere around us, nature always provides balance. Even an artificial environment like an aquarium is not exempt from this rule. If you are planning to set up a new aquarium or add a new fish to your existing one, you should first know how these environs thrive.
Without proper knowledge or guidance, adding a new fish to an already stable aquarium may cause new tank syndrome which is quite distressing for the aquarist. It is abhorred by enthusiasts and often leads to death.
Although you may only see a fish in your aquarium, there are actually a lot of other creatures involved in the stability and life in your aquarium. As in the wild, there is also a delicate balance in the aquarium that needs to be maintained.
One of the most important processes a good informed aquarist should know is the nitrogen cycle. Nitrogen, the most abundant element in the atmosphere, is also present in almost every tissue of every living creature. It also is abundantly excreted by fish in the form of ammonia (NH3). Even in small concentrations, ammonia can be poisonous to fish, damaging the gills, inhibiting oxygen supply and eventually causing paralysis or death.
Luckily, there are heterotrophic (waste-eating) bacteria that break ammonia down. First, Nitrosomonas breaks down ammonia to nitrite (NO2) which is still poisonous to fish. Then Nitrobacter steps in. It further metabolizes nitrite and excretes nitrate (NO3) which is quite harmless to fish (but food for algae). Without these two bacteria, your fish will suffocate and die in a matter of days. Talk about a fish drowning!
Fish and microorganisms in an aquarium create a delicate balance. Unfortunately, these two bacteria multiply slowly at first. When there are too many fish in the tank, the microorganisms cannot catch up before the fish suffocate.
On the brighter side, there are ways to jumpstart this cycle and balance it for the new fish before it arrives. This is what aquarists call cycling. What usually happens is that aquarists introduce an ammonia source on behalf of the new fish and wait for the heterotrophs to catch up. Once the heterotrophs balance out the excess ammonia, then the new resident can come in.
An example of cycling is using two or three cocktail-sized dead shrimps per new fish into the aquarium. When the shrimps start to decompose, it will release ammonia into the water. Then you should watch ammonia and nitrite levels increase, peak, and gradually decline. Once ammonia and nitrite levels reach zero, the new fish can be accommodated.
Introducing hermit crabs in an aquarium is another example of cycling. Buy a couple of hermit crabs as interim residents for your aquarium. Like fish, they excrete ammonia as waste. The microorganisms will then have accelerated growth until they have enough population to support the hermit crabs. Once ammonia and nitrite levels have zeroed, replace the crabs with the new resident fish. This type of cycling is more natural than the cocktail shrimp method. Needless to say, hermit crabs are also infinitely more exciting than a couple of dead shrimps.