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Fishless Cycling  

by Chris Cow, Ph.D.

Nitrogen cycle
Generalized diagram showing the progression of ammonia, nitrite, and nitrate pulses after a new tank is set up (as fish wastes start to decompose). If fishless cycling is performed, the necessary beneficial bacteria are introduced and controlled amounts of ammonia are added as described in the article below. Diagram by Erik Olson, reproduced here with his permission from The Aquaria FAQ

      Note from M.K. Oliver, Webmaster of this site:   Failure to understand the nitrification cycle (often called the "nitrogen cycle") is one of the most frequent serious mistakes of novice cichlid keepers. If a newly set up aquarium is not properly "cycled" before stocking it with fish, they will very probably die of ammonia poisoning, and the aquarist may decide that keeping cichlids is not worth the effort. This is avoidable! Traditional cycling works, but results are highly variable, and fish may suffer or die in the process. Chris Cow, an organic chemist, invented an alternative, "fishless cycling," several years ago. This seems to be a significant improvement over traditional cycling methods. In the interest of promoting this faster, safer, and more humane technique, the following article is reprinted in full (with slight editorial changes) from the original posting of Dr. Cow's follow-up article.

W hen I wrote an article on fishless cycling in January 1999, I had no idea that it would gain popularity on the Internet so quickly. What started as a very obscure idea has become a viable alternative for many newcomers and more experienced hobbyists alike, thanks mostly to promotional support from a few individuals who tried the idea out and then recommended it to others.

When I first wrote the article, I had tried this method out on two of my own tanks; since writing it, I've received feedback from literally dozens of individuals, and I now have a much better understanding of the process itself, and some potential pitfalls.


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. In practical terms, this means that your tank cycles faster (reports of anywhere from 10 days to 3 weeks, depending on the fish tank... compared to average of 4-6 weeks for traditionally cycled tanks), and that you can fully stock a tank when the cycle is complete. This latter 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. Of course, another big advantage is that no fish are subjected to high ammonia or nitrite concentrations, eliminating mortalities and ammonia/nitrite related illnesses which frequently occur in new tanks.

Alternative Recipe

While the original recipe works quite well (4-5 drops NH3 / 10 gal / day until nitrite peaks, then reduce to 2-3 drops / 10 gal / day), it does NOT take into account varying concentrations of ammonia that are available. ACS [American Chemical Society] grade ammonia, which I was using, is ~28% NH3, while most household cleaner grades vary from 4-10%, a fairly wide variation in concentration. 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. With that in mind, I'd like to propose a different recipe, which was suggested by D_Man and others (thanks!): Add ammonia to the tank initially to obtain a reading on your ammonia kit of ~5 ppm. Record the amount of ammonia that this took, then add that amount daily until the nitrite spikes. Once the nitrite is visible, cut back the daily dose of ammonia to 1/2 the original volume. One advantage of this method is that the ammonia spike occurs immediately.... When adding 4-5 drops / 10 gal / day, it could take 4-5 days before the ammonia reaches the same levels. This should result in an acceleration of the entire process, though by how much (on average) remains to be seen.

Sources of Bacteria

While it is probable that the bacteria required for the conversion of ammonia and nitrite to nitrate exist at very low levels in most uncycled tanks, it greatly accelerates the process to inoculate the tank with a large dose of healthy bacteria to get things started. Good sources of beneficial bacteria are ranked from best to least:

  1. Filter material (floss, sponge, biowheel, etc.) from an established, disease-free tank.
  2. Live plants (preferably potted, leave the rockwool on until cycling is finished). Crypts or amazon swords are good choices, and not too demanding.
  3. Gravel from an established, disease-free tank. (Many lfs [local fish stores] will give this away if asked nicely.)
  4. Other ornaments (driftwood, rocks, etc.) from an established tank.
  5. Squeezings from a filter sponge (any lfs should be willing to do this...)

There are also a number of commercial bacterial supplements (Cycle, Stress-Zyme, etc.) available. In my humble opinion, without getting on a soapbox, these have very little to no effect, and are best left on the shelf. If you want to try it, go ahead, but I believe that any of the above options will be more effective, and most if not all of them will be cheaper.

Sources of Ammonia

T he most difficult part of the fishless cycling procedure, according to many postings on the message boards, involves finding a good source of ammonia. Ammonia used should be free of surfactants, perfumes, and colorants. Unfortunately, not everyone has access to ACS grade ammonium hydroxide. 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. To paraphrase RTR: 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; ammonia with surfactants will foam, while good ammonia will not.

Water Changes and Ammonia-Removing Chemicals

A large water change (50-70%) should be done before adding any fish to the tank to lower nitrate levels, which can be a pain to bring down later. When changing the water during a fishless cycle, do NOT use dechlorinators that also sequester ammonia, such as the very popular Amquel. I have heard
from at least one individual who did everything right with regard to cycling her tank using this method ... the tank cycled quickly, then she did a water change, then added a reasonable fishload the following day with more than adequate filtration, and observed both an ammonia and a nitrite spike. The only explanation that I could think of after questioning her extensively led back to the Amquel. In a normal, established fish tank, the ammonia is being generated nearly constantly. In a fishless cycle, however, the ammonia is added as a daily dose. In my opinion, it's conceivable (though not really provable unless a lot more people experienced identical problems) that the Amquel temporarily deprived the bacteria of its food source, causing a minor die-back in the colony at the worst possible time... right before adding her fish. To be on the safe side, use a simple chlorine/chloramine remover which does not affect your ammonia levels.

By similar logic, any other ammonia-removing chemicals (e.g., Ammo-lock) or resins (Amrid) should also be avoided while cycling; they will affect the cycle, extending its duration or otherwise adversely affecting the bacterial colonies.

Too Much Ammonia?

It IS possible to add too much ammonia to the tank (generally several times the amounts suggested in either recipe), as some individuals discovered by mistake (thanks Boozap). What happens in this case is that the ammonia will spike very far off the chart, then the nitrite will spike as well (also way off the chart), and it will continue to spike for a very long time. Why? There are a couple of possibilities. The first is that the filter media and surfaces in the tank or oxygen levels are simply insufficient to grow and maintain a bacterial colony massive enough to convert all of the ammonia and all of the nitrite to nitrates. Another likely possibility is that the ammonia levels are high enough to inhibit growth (through a biofeedback mechanism) of the bacteria rather than promoting it. The solution is quite simple, however. If you realize that you've added way too much ammonia, simply do a water change, or if necessary a series of water changes, to bring the ammonia and/or nitrite levels back into the readable range on your test kit. Then proceed as normal with daily additions of ammonia until the tank is cycled.

Other Uses

Fishless cycling is also very applicable to hospital/quarantine or fry growout tanks. When not in use, a maintenance dose of ammonia (e.g., 2-3 drops/10 gallons) can be added daily to keep the tank cycled and ready for new fish indefinitely. Simply stop the addition the day before you want to buy your fish, take ammonia and nitrite tests to be certain that the levels are still zero, and do a water change to reduce nitrates.

Future of Fishless Cycling

The future of this method is up to those of you out there who have tried this and like the method. If you do, please continue to promote it, whether on the Internet, at your lfs, or at fish club meetings. Feel free to print off this or the original article for educational purposes; the more people that know about this method the better. I'd be very happy if lfs caught on to this idea as have fishkeepers on the Internet, and recommended it to newbies as a safer, cheaper way to do things. In my opinion, the number of people that didn't get frustrated in the early stages, and thus continued with fishkeeping, would make it worth their while. Please continue to give me feedback on the method, or to ask any questions you may have about it.

Written by Chris Cow
Ph.D. Organic Chemistry



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Last Update: 20 February 2009
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