This is an article I wrote on Fishless Cycling. It appears on the Age of Aquariums web site: Age of Aquariums
I wouldn't dream of cycling a tank any other way! Since this article was written, I've also cycled a 55 gallon planted tank using this method, and another 10 gallon planted. The 55 gallon took 6 weeks. The new 10 gallon took 4 days. I hope someone finds it useful.
When setting up a brand new tank, the most important step involved is the so-called cycling of the tank: that initial period (usually 4-6 weeks) where a large colony of beneficial bacteria will grow and help process the wastes produced by the tank's inhabitants. During the cycle, certain toxins - mainly ammonia and nitrite - rise to very high levels until the bacterial colony is large enough to process them. Since the existence of fish wastes have always been considered the starting point of a cycle, traditionally the cycling process would involve initially adding a few tougher fish (such as Zebra Danios) and allowing their waste to cycle the tank. This method is indeed effective, however, it is very stressful for even the strongest fish. Ammonia is highly toxic, and its effect on the fish's gills could be compared to shampoo in your eyes...it "burns"! Worse yet, it will also cause permanent damage to the very tissues that allow the fish to breath. This damage is so serious that a significant fraction of fish used to cycle a tank die during the cycling period itself, and even those who do manage to survive the cycle have their life expectansy greatly shortened, often dying within the next few months instead of living a happy life of 3-10 years that almost all fish are capable of. In many cases, people also end up getting stuck with a few fish that they really didn't want to have in the first place, but bought them anyway because someone told them they were "good for cycling", if such a thing exists.
In more recent years, now that we've understood in much more detail the chemistry and biology involved in fish tanks, we have come to realize that the only ingredient really necessary to perform the initial cycle is not the fish waste, but the ammonia itself! So we have come to the point where we can question the necessity of subjecting our poor fish to the "torture" described above. The answer, as I hope to show here, is that we really don't.
A growing trend in tank cycling is the method called Fishless Cycling. This method is more humane than the traditional way because, as the name implies, no fish are used. It's very simple to do, and I've cycled many tanks this way, and love the results. Tanks cycle in their own time and you cannot really do much to force the process, however, in my own experiences I have found the fishless method to be faster than the traditional method. My 100 liter (29G) tank was cycled using traditional methods and took a full 6 weeks. The rest of my tanks have cycled thusly:
40 liter (10G) non-planted - 7 days
70 liter (18G) planted tank - 2 weeks
23 liter (6G) planted - 10 days
23 liter (6G) non-planted; 3 weeks
So how does this method work?
You'll need only two things: a source of ammonia and a test kit for nitrogen compounds. You'll need one for testing ammonia and one for nitrites. Having one for nitrates would be helpful as well, but I've found the other 2 to be the most important for this process (careful, those last two names are very similar and easily confused, but they are totally different compounds in terms of their role in the nitrogen cycle and the damage they can do to your fish). These test kits can be readily found for sale in Local or Internet Fish Shops, and even in major supermarket chains, depending on where you live. The ammonia can be just plain household ammonia, preferably the non-sudsing kind which is usually colorless. It's good to have a graded recepient to measure the amount of ammonia you're adding, and also have all of the standard gear you'll eventually need to do water changes already at hand; buckets, siphons, etc. Some people, such as myself, like to use commercial products such as Cycle or StressZyme when doing fishless cycling. Others feel that these products are a waste of money. It's all personal preference, and you can definitely cycle a tank without using any bottled bacteria since they exist everywhere, even floating in the air.
If you have at least one other established tank in your house, it would be helpful for the cycling process to take some gravel from the established tank, put it in a sock or a segment of pantyhose, knot it, and then put it in the bottom of your new tank to help seed it with 'good' bacteria. Also, adding some clean water from your established tank to the new tank will aid in the cycling process. Still another helpful trick is to hook up the new tank's filter to an established tank and run it for about a week before installing it on its final tank. Or, you can simply place the new tank's filter media in your old tank for about a week, then put it in the proper filter before you begin the cycling process. If this is your very first tank, and you do not have access to someone else's established tank to use these ideas, don't worry. You can still fishless cycle.
OK, the decorations are in place, the tank is filled, the water has been treated with any required water conditioner and the filter is chugging along nicely. Raise your temperature to 30-32°C. This will help speed the process, but you do not have to wait until the temperature reaches these levels before you add ammonia. You can add it at any time after the tank is filled. If your tank is going to be planted, you can and should add the live plants during the cycle (unlike fish, plants actually like ammonia as long as its level isn't way too high), but then lower the temperature down to 25-27°C...most plants don't like warm water.
Get your ammonia test kit ready! Add some ammonia. Start small, only a couple of teaspoons for a small tank around 40 liters (10G), or use about a 1/2 cup for large tanks around 200 liters. Let it sit for an hour or so to allow the ammonia to circulate. Test. Your goal is to get a reading of about 3 to 5 mg/L (or ppm, it's the same). If you have zero ammonia readings, add a bit more to the water. Not too much...you are using a toxic chemical afterall. Let the tank sit. Test. Keep testing and adding SMALL amounts of ammonia until you get a reading in your test kit. When you have a reading, you may want to add Cycle or StressZyme if you've decided to use these products. Keep in mind that the commercial bacteria is bottled in a non-toxic form of ammonia, so if you use them, your ammonia readings will be higher than if you hadn't used them. This is OK. You may discover your ammonia levels will go over 6 ppm (parts per million), which is fatal for all fish, but this is OK too, since you don't have any fish in the tank.
As soon as you notice high levels of ammonia, stop adding ammonia to the tank. Now is the part where your patience is tested! Let things run their course, and keep testing the water. Once a day is fine, or once every other day. After a few days, you can begin testing for nitrites as well as ammonia. If you aren't getting any readings for nitrites at all, that's OK. These things take time. Don't do any water changes yet, and continue to let everything sit. When your ammonia starts dropping, you should definitely be able to read some nitrites. After the ammonia reading drops to zero, start adding just a little bit more ammonia again every day, just a teaspoon or so...not enough to force the reading above zero again, but just enough to keep the newly grown "ammonia-eating" bacterial colony happy.
When you notice your nitrites are spiking (reach a maximum and start going down) you'll know you're almost done! In my experience, the tanks have finished cycling within 3 or 4 days of the nitrite spike, but the amount of time will vary for everyone. When your nitrites drop to zero AND your ammonia is zero, then your tank is cycled! Do a large water change, around 50% to 80%. Do not change any filter media, and do not vacuum the gravel during this water change. Refill with fresh, conditioned water, and lower your heater to 25°C or whatever temperature is best for the fish you intend to keep. Let everything chug for several hours or overnight. Test the water for both ammonia and nitrites again. If everything is still zero, the tank is cycled and you're ready to add fish!
Although in certain more advanced fish setups it's actually better to add all the fish at once, for the general community tank and especially for beginners, it's always best to add fish slowly - only a few per week - regardless of the cycling method, so that the bacterial colony has time to continue growing in accordance to the increasing production of fish wastes. Adding all the fish at the same time will often cause ammonia/nitrite to spike again even in a cycled tank, which may lead to a general wipe out of the entire group within a few days, or cause an outbreak of diseases due to their weakened condition. The rate of addition should be such that you'll complete your full population at least one month after the end of the cycle.
I hope I've convinced you to go easy on your fish during the beginning of your tank, they'll retribute by letting you watch them live a happy, healthy life for many years. Good luck!