The Basics (Nitrogen Cycle)
The "Nitrogen cycle" (more precisely, the Nitrification cycle) is the biological process that converts Ammonia into other, relatively harmless Nitrogen compounds. In nature, the volume of water per fish is extremely high, and waste products become diluted to low concentrations. In aquariums, however, it can take as little as a few hours for Ammonia concentrations to reach toxic levels.
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When organisms give off wastes they excrete Nitrogen in some chemical form or another. For example, people rid themselves of it in the form of urea, while birds use uric acid. However, fishes and many other marine organisms give it off in the form Ammonia
(NH3). Ammonia is also produced and released when the tissues of deceased organisms and fish foods decay due to bacterial action.
The problem for the hobbyist is that this Ammonia is highly toxic and if it is allowed to build up in an aquarium the end result will be the death of the inhabitants 10 times out of 10.
The solution to this problem is a number of bacteria that "eat"
Ammonia and use it for their own food source. Bacteria like
Nitrosomonas, Nitrosococcus, and Nitrosospira will use the energy stored in the Ammonia molecule, then release the Nitrogen left over from the process as another form. In this case, it is combined with
Oxygen and re-introduced to the environment as Nitrite (NO2).
Nitrite is also a deadly poison to tank inhabitants but there are other types of bacteria which will then use the Nitrite for food.
These bacteria, Nitrobacter, Nitrococcus, and Nitrospira, add another Oxygen to the nitrite molecule and acquire energy in the process. The nitrite is thus converted to yet another Nitrogenbearing compound called nitrate, which in reasonable quantities is non-toxic/"harmless" to tank inhabitants (NO3). We still need to control NO3 or one will experience problems with algae (e.g., Algae
Bloom). Nitrate levels are controlled through periodic water changes which are usually ~15%-25% on a weekly basis.
The Basics (Fishless Cycle)
For many years, the common method of cycling a tank had been to set everything up, then add a few hardy or "disposable" fish, then wait 4-6 weeks until the bacterial colonies which convert Ammonia
to Nitrites and then to Nitrates have become established. It is very common at this point for the stress caused by toxic Ammonia and/or
Nitrites to kill some or in extreme cases all of your starter fish, no matter how hardy they're supposed to be. In addition, it's a well known fact that the damage caused by high Ammonia levels to the gills of a fish is, to some extent at least, permanent. The second method - the purpose for my writing this - avoids the stress on the fish by artificially adding Ammonia, which we call "Fishless Cycle".
The advantages of this process over the traditional method of cycling a tank using a few small, hardy fish to get the bacterial colonies up and running all result from "front-end loading" the tank.
The amount of Ammonia added is far above that generated by a reasonable number of cycling fish, resulting in faster growth of the bacterial colonies, and larger colonies when you're finished.
Another great advantage of fishless cycle is the flexibility of fully stocking a tank after the cycle is complete. This point is of particular interest to keepers of African cichlids or other aggressive fish. If these fish are all added together as juveniles, they're much more tolerant of each other than if they're added in small groups after the first
