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Fishless Cycling

 Age of Aquariums > Aquarium Articles


When setting up a brand new tank, the most important step is the cycling of the tank: that initial period--usually four to six 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 burns. Worse yet, it will also cause permanent damage to the very tissues that allow the fish to breathe. The resulting damage is so harmful that a significant fraction of fish used to cycle a tank die during the cycling period itself. Even those who do manage to survive the cycle have their life expectancy greatly shortened, often dying within the next few months instead of living a happy life lasting, typically, three to 10 years. In some cases, people end up with a few fish that they really didn't want in the first place, but bought regardless because they were told the fish were good for cycling.

Now that we better understand the chemistry and biology involved in aquariums, 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. We must now question the necessity of subjecting fish to the torture involved in cycling new aquariums. The answer, as I hope to show here, is that we really do not.

A growing trend in tank cycling is a 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, and I've cycled many tanks this way with excellent results. Tanks cycle in their own time and you cannot 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 fishless cycling work?

You'll need only two things: a source of ammonia and a test kit for nitrogen compounds--one for 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. A word of caution: nitrates and nitrites look very similar and are easily confused, but they are totally different compounds in terms of their role in the nitrogen cycle and the damage they can inflict on your fish. Test kits are easily found for sale in local or in online fish supply shops, and even major supermarket chains, depending on where you live. The ammonia can be just plain household ammonia, preferably the non-sudsing kind that is usually colorless. It is good to have a graded recipient, such as a measuring cup, 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 like to use commercial bacteria 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 bacteria 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 beneficial bacteria. Also, adding some clean water from your established tank to the new tank will aid in the cycling process. Another helpful trick is to hook up the new tank's filter to an established aquarium and run it for about a week before installing it on the new tank. Or, you can simply place the new tank's filter media in your old aquarium for about a week, then put it in the proper filter before you begin the cycling process. If this is your very first aquarium, and you do not have access to someone else's established tank to use these ideas, do not fret; you can still fishless cycle.

Moving along--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 too high. Lower the temperature to 25-27°C because plants do not like warm water.

Get your ammonia test kit ready! Add some ammonia. Start with 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 fine. You may discover your ammonia levels will go over 6 ppm (parts per million), which is fatal for all fish, but this is fine too, since you do not have any fish in the tank.

As soon as you notice high levels of ammonia, stop adding ammonia to the water. 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 fine. 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 dropping) your aquarium will be nearly ready! 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 are 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 is 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. This kind of event may lead to a general wiping out of the entire group within a few days, or cause an outbreak of diseases because of the fish’s 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 when establishing a new aquarium. They will repay you by living a happy, healthy life for many years. Good luck!

Shawna McGregor-King
Reader Comments Comentário

This is an excellent article, nice work. The only thing I can say to ANYONE about to attempt the fishless cycle is: check the ingredients list on your ammonia. Don't use ammonia that contains any sort of soap, especially the ambiguously named surfactant. If there is any question at all, ask on the forums. Using ammonia containing surfactant will basically ruin your tank. Good luck!

Contributed by Liszie F.

I actually tried this (more or less) with the 100 L I am in the process of setting up. I didn't actually know this was a standard practice, but after the common dreadful experiences with new tanks, I thought I would try something different in order to spare the fish with this one. I never considered actually adding ammonia directly, but I set the tank up with a few small plants (after conditioning the water), and added Bio-zyme and a little fish food every day or two to decay and generate the ammonia. I then tested for ammonia, nitrite, and nitrate regularly. The tank underwent the normal ammonia spike, then a nitrite spike, and now (about 2 weeks after set-up) ammonia is at 0 ppm, and nitrite is at very nearly 0 (<.25 ppm). My Bio-Wheels have darkened noticeably, the plants seem to be doing okay, and I'm about ready to add fish. However, I'm not claiming success until I've introduced a few fish and all goes well. We'll see...I think it's a great idea though, if it works, and definitely worth a try for all the reasons presented in the above article.

Contributed by C. Simpson

I have been in this hobby for many years, but only recently understood the need and the process of cycling. My wife and I moved into our new home two years ago and underwent of starting our tank again. My first time at fishless cycling and it worked! I never used commercial grade ammonia, I used frozen prawns (meant for human consumption), peeled them and placed 15 in a nylon stocking. I chose this method because I had to leave for a business trip for about 7 days. This allowed the ammonia, and nitrite to build up in the tank while I was gone. The rest of the process I followed just like above and it worked. I kept a log, but I don't have the document anymore. I suspect it cycled within 4 to 5 weeks.

Contributed by Ron H.

I just wanted to let you know about the experience that I've had so far in regards to fishless cycling. I found out about fishless cycling quite by accident. All the advice I had gotten from people suggested that I throw a few cheap fish into my tank and once the cycle is finished, just get the ones I really want and flush the cheap fish down the toilet. Then I saw this site and a few other similar to this. It sounded very interesting, so out of curiosity I thought I would give it a try. After all, the cheap fish don't deserve to suffer just for our pleasure. I set up my 280 liter tank and found the pure ammonia at a hardware store. I added enough ammonia to my tank to get it up to about 5 ppm. After about 2 weeks, I still had nothing but a lot of ammonia, so I went and got a large raw shrimp from the grocery store, put it in a knee-high stocking and added it to the tank. The ammonia level gradually went up to about 7 ppm. About 2 weeks after adding the shrimp, (4 weeks from beginning) I was excited to see nitrites developing. The whole process has been almost 5 weeks now, but my nitrites are about 5 ppm and and my ammonia has dropped to about 1.5 ppm. It has been fun for me, as I have the test kits with the test tubes and it reminds me of my chemistry class in high school. I intend to have both plants and fish in my aquarium, so I have used the wait time to research the plants and fish that I would like to keep in my aquarium. Hopefully it won't be long now until I am buying my fish. I'm just excited to see that it really works!

Contributed by Pam

I recently tried fishless cycling to start my new 40 liter tank. I'm new to fish care and I didn't know how much pure amonia to put in, so I dumped in 2 teaspoons. It's good there were no fish in the tank as it raised the level off the chart. I waited and in 3 weeks the amonia level was down to zero. I began feeding the tank 1/4 teaspoon of amonia every day and it processed it in 24 hours. My nitrite and nitrate levels were off the chart by this time. I waited another 2 and half weeks, which seemed like forever, and the nitrite was down to zero. I changes about 60% of the water and that brought the nitrate down to safe levels too. I let the tank settle for a day and checked the water again before adding 3 cherry barbs. All seems well so far. In a couple weeks I plan to add some neon tetras that I have in quarantine in a small, established tank. I'm so glad I cycled the tank this way instead of putting any of my fish at risk.

Contributed by Carolyn S

I used the guidance on this page with my 125 litre Juwel freshwater aquarium. I used household ammonia and the Nutrafin test kit. Having a small 25 litre tank I also squeezed the sponge from the filter into the Juwel filter twice to help things along. The tank cycled in 16 days - Ammonia 0, Nitrite 0 and Nitrate 5 before the 30% water change and a pH of 7.5. Tested again after 24 hours of the water change - readings still fine - added first 4 fish. Thanks for the excellent guidance! My previous tank (25 litre) bought from a national retailer was run for week with Nutrafin cycle added as per the shop staff instructions. When 4 fish died I took the water in for testing (at their request) - Nitrates too high - my first introduction to water chemistry and the last time that any of my fish suffer in a cycle!

Contributed by Laurie Kenny

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