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What is Fishless Cycling?

By R. Kayne
Updated May 23, 2024
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Fishless cycling is the process of cycling a tank, or establishing a biological filter in a fishless aquarium.

The biological filter, or nitrogen cycle, naturally occurs over the first 4-6 weeks after a tank is set up. Cycling a tank is necessary to keep the water free of pollutants and the fish healthy, but while the filter is establishing itself, fish are exposed to toxic levels of ammonia and nitrite and often succumb to disease and death. Fishless cycling removes fish from harm by establishing the biological filter before fish are introduced to the aquarium.

Normally, when fish are added to a new aquarium, even though the water is clean, ammonia begins to build immediately, as fish release ammonia through breathing and waste. Uneaten food, and decaying plants also create ammonia. However, ammonia is necessary to start the nitrogen cycle -- the first step in establishing a biological filter.

In January 1999 Chris Cow who holds a PhD in organic chemistry, posted a paper online regarding fishless cycling. He got the simple idea to add ammonia to a fishless aquarium in order to get the nitrogen cycle going. This not only saves fish, but it saves the aquarist from stress as well.

At this point all of the normal phases of the biological filter can be triggered without worry. Ammonia levels can soar, followed by toxic levels of nitrite, until, finally, both become neutralized. With the tank fully cycled any ammonia created by adding fish will now quickly be converted to harmless nitrate by the established colonies of positive bacteria that make up the biological filter.

Chris Cow's experiment worked wonderfully, and has been used by countless aquarists since.

To use the fishless cycling method simply add 4-5 drops of pure ammonia per 10 gallons of water to a fishless tank. Do this once a day until nitrite becomes measurable. At that point cut back to 2-3 drops per 10 gallons of water, per day. When nitrite and ammonia are both at zero, and nitrate is measurable, the tank is cycled. A substantial water change of 25% or more is advised, then your tank is ready for a full complement of fish.

Only pure ammonia, without detergents or colors, should be used. Ammonia with additives will foam up when you shake the bottle. Pure ammonia will not foam.

A simple declorinator/de-chloraminer is recommended while cycling fishless, rather than products like Prime or AmQuel that sequester ammonia by converting it to non-toxic ammonium. The concern is that sequestered ammonia may be harder for bacteria to process, and therefore could slow down the establishment of the bacteria. Once the biological filter is established, adding products like Prime and AmQuel are excellent and will not hamper the biofilter.

Biofilter bacteria thrives at about 80 degrees Fahrenheit (27 degrees Celsius). If you turn up your tank's heater while cycling fishless, the tank can cycle in as little as 7-10 days. Be sure to turn the heater back down when the nitrogen cycle is complete, and allow the water time to reach the desired temperature before adding fish.

The great advantage of fishless cycling is that it is humane and stress-free. Another great advantage is that you can add a full complement of fish at once, because the biofilter is fully established. This is especially significant when keeping territorial fish that are better added all at once.

Other advantages include keeping a quarantine or hospital tank cycled in case of emergency, by simply adding a drop of ammonia to it each day to simulate the presence of a fish.

Any aquarist who has gone through the difficulty of cycling a tank with fish, will surely appreciate the fishless cycling method!

Sports n' Hobbies is dedicated to providing accurate and trustworthy information. We carefully select reputable sources and employ a rigorous fact-checking process to maintain the highest standards. To learn more about our commitment to accuracy, read our editorial process.
Discussion Comments
By anon89054 — On Jun 08, 2010

It would be best to get a 30 gallon kit and set the 30 gallon's filter in the 10 gallon with biological media to filter for a week or so. Then set up the 30 gallon tank, first putting in new gravel then layering on old gravel from the 10 gallon tank. If you have a filterless tank, most of the ammonia-consuming bacteria will be living on the gravel.

It would be best for your fish to use this process rather than waiting for the 30 gallon tank to cycle properly. Make sure when transferring the old gravel to not let it dry out; this will kill the bacteria.

By anon26621 — On Feb 16, 2009

It's wonderful to hear you have taken your little goldfish's care to heart.

Goldfish are one of the "dirtyist" types of fish you can get, meaning they create a lot of waste that turns to harmful ammonia in the water. They can also get large, if they are well taken care of and live long enough, so your idea for a 30g tank is a good one.

You *should* get a filter for them, and one that removes waste as well as keeps the biological filter healthy to transform all that ammonia into nitrate. It will cost a little money but the *great* news is that you will only have to change 10-15% of the water every couple weeks, and the fish will be much happier.

While there are small, cheap filters available, they will not do much good for three goldfish and you'll be wasting your money and become frustrated. You will thank yourself in the long run if instead you get a canister filter that will sit below the tank, under the stand. It sucks water down into the canister by gravity feed, then pumps it back up into the tank through a spray bar that disturbs the water's surface, adding oxygen. The canister is filled with several types of media to not only strain the water of impurities, but grow good biological bacteria. I think wiseGEEK has an article on canister filters.

Canisters are by far the best and most efficient filters for removing waste (needed for goldfish) and keeping a healthy biological filter going. They are also the least trouble, as they only need occasional cleaning (maybe every few months to once yearly depending on the size of the canister vs your fish load), and they are the only filters worth every penny they cost, imo, after 20 years of fishkeeping.

A large hang-on-tank filter would be next in line, but will require weekly or bi-weekly cleaning and won't do the job of a canister. They cost about half of what a canister costs. ($50 vs $100.) I know it sounds like a lot of money, but it's just one outlay that lasts years, and totally worth it for all the pleasure a tank and fish can bring to a house... not to mention the trouble/time it saves you.

And yes, you would cycle the 30g first, fishless with the filter installed, then add the fish once the tank is cycled.

Another good addition, if your pocketbook could handle it, would be an undergravel filter. These are inexpensive. They consist of a plastic filter plate under the gravel, with a plastic feeder tube on one or both sides, rising to a small motor pump (called a power filter) that sucks water up the tube, drawing tank water down through the gravel. This keeps the biological bed in the gravel healthy, while drawing waste down into the bed where it can't be seen floating about, and can be transferred into harmless nitrate. When you clean the tank (removing your 10% water), you just vacuum the gravel by pushing the vacuum tube down through the gravel against the plates. This cleans the gravel and pulls waste out from under the plates too. Draw that dirty water into a bucket and discard...then top off the tank with fresh water that has been treated to remove chlorine, etc.

By puppysmama — On Feb 11, 2009

My son won a comet goldfish last summer. We fell in love. Now he's 4 inches. We upgraded to a 10 gal, tank w/o filter and added another comet and an Oranda Goldfish. Now we're stumped. And of course dealing with mucky water and 3 gulping goldfish.

Will adding a simple filter be enough for these three for a long time?

If I move to a 30 gallon tank, do I need to do Fishless cycling and just keep them in the unfiltered 10 gal and change the water daily?

Or would it be best to set up the 30 gallon and put them in and watch the levels from here on out?

I'm totally new at this, but in love with 3 little goldfish! Thanks.

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