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.
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 Nitrogen-bearing 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 fish have established their territories.
Pure Ammonia as initial element
In order to properly cycle a tank, all that's required is the filter media, water movement to supply Oxygen to the bacterial colonies, an introduction of the right type of bacteria, and a source of Ammonia. The best and most efficient source of Ammonia is pure Ammonia. The household cleaning variety is perfect for this use, but make sure that it does not contain any additives or perfumes before using it! Ammonia used should be free of surfactants, perfumes, and colorants. Always read the ingredients on the bottle. The best sources for Pure or Clear Ammonia are discount grocery stores or hardware stores. Often, the no-name brand is the stuff you're looking for. Some other people have reported success with the following brand names of Ammonia: Top Crest or Whirl Clear Ammonia. If it doesn't list the ingredients or say Clear Ammonia (or Pure Ammonia or 100% Ammonia, or Pure Ammonium Hydroxide), then leave it on the shelf and look elsewhere. Shake the bottle if you're not sure about it because Ammonia with surfactants will foam, while "good" Ammonia will not.
Other bacteria sources and ways to speed up the process
I strongly recommend the use of other sources that contain a sufficient amount of "good bacteria" in order to noticeably speed up the cycling process.
Filter material (floss, sponge, cartridge, biowheel, etc.) from an established, disease-free tank.
Gravel from an established, disease-free tank. (Very efficient source of bacteria. Possibility of getting it from friend or LFS)
Other ornaments (driftwood, rocks, etc.) from an established tank.
Squeezings from a filter sponge ("sponge mud" is very rich in bacteria)
Live plants (if your tank is heavily planted, the chances are you won't see an Ammonia or nitrite spike if you track these parameters when cycling. In fact, the only indication that your tank has cycled may be the appearance of Nitrates. Even then you may not get a reading: heavily planted tanks with a light to moderate fish load often test zero Nitrates, since the plants take up some of the Ammonia before the bacteria convert it)
Increase of temperature of the tank. Chemical reactions are accelerated at higher temperatures which will cause the bacteria to divide faster. Be careful not to raise it too much. Over a certain point, bacterial growth is impeded. I've found the mid- to high-eighties work well.
Others would add 2 more sources which I find not as efficient and in some cases worthless.
Addition of established water.
Addition of commercial bacterial supplements (exception of Bio-Spira ??)
There is no general recipe for fishless cycling. Since different brands of Ammonia have different concentrations, there is no formula for "x amount of Ammonia per gallon," you just need to keep testing. ACS grade ammonium hydroxide, for example, is ~28% NH3 while most household cleaner grades vary from 4-15%. Bottles that have been left open for long periods of time will be lower in concentration, as the NH3 gas escapes back into the atmosphere.
Currently there are two "good" recipes available and tested by other hobbyists.
1st method. 4-5 drops of NH3 / 10G / daily until NO2 peak, then reduction to 2-3 drops of NH3 / 10G / daily (this recipe uses ACS grade ammonium hydroxide ~28% NH3)
2nd method. This one is used by probably 99% of hobbyists. Addition of X amount of NH3 drops until Ammonia level of 5ppm is achieved. This X amount of drops has to be added daily until NO2 spike. Afterwards follow up with ½ X (from previous step) amount of NH3 drops daily until NO2 is 0ppm causing NO3 peak. ~50% water change should follow -> cycled tank. (this recipe uses regular Pure Ammonia 4-15%)
Once the tank has been cycled, the bacterial colony created by this method can handle a large bio load immediately. The amount of Ammonia produced directly relates to the amount of bacteria that will be in the tank (Bio Load). The amount of Ammonia added to the tank during the cycle is significantly higher than what would be contributed by a small number of hardy fish, therefore, a much larger, healthier bacterial colony exists at the end of the cycle using Ammonia than would if you used fish.
While posting on various forums I noticed many general comments/questions regarding fishless versus fish cycling. Many of them could not be answered by others because of a lack of knowledge about a given tank and the existence of so many possibilities. Someone's tank could be cycled in a week while another could take two weeks because of the presence of more established media in the first one. One should follow the above formula and general fishless cycling should take anywhere from 5-14 days. How fast one can achieve a successful cycle is strongly related to how much established media one can provide.
Water changes should take place after cycling is done and NO3 is at very high levels. I don't see any purpose for water changes while in the process of fishless cycling. Products such as Amquel, Ammo-lock or Amrid are counterproductive and should also be avoided while cycling. Amquel could temporarily deprive the bacteria of its food source, causing a minor die-back in the colony. These products will affect the cycle, extending its duration or otherwise adversely affecting the bacterial colonies. The "Proper amount" of Ammonia is necessary for cycling and less/more amounts are not a possibility.
I have drawn upon the knowledge and experience of other hobbyists as well as my own in an effort to gain an audience for fishless cycling. In my opinion Fishless Cycling is not the only way to start a tank but it's definitely a lot faster, safer and a less expensive way to get into this hobby. □
“Fishless Cycle” by Chris Cow Ph.D. Organic Chemistry
“Nitrogen Cycle” by TheKrib
“Biological Filtration Basics” by James Fatherree