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How to Fishless Cycle an Aquarium.

Updated: Jun 12, 2022

Every aquarium owner eventually finds themselves at the crossroads of fish tank cycling. Do you go down the path of fishless cycling with liquid ammonia or fish flakes, or do you add fish right away? Do you seed your tank with established biological colonies, and how much should you use? For those of you who are seeking answers to these questions, follow me and I'll guide you down the path I've traveled many times and explain why its best to avoid the others.

What Does Cycling and Aquarium Mean?

Cycling an aquarium refers to initiating the nitrogen cycle in your fish tank. This chemical and biological process is one of the most crucial elements to maintaining a healthy ecosystem. The cycle begins and ends with the element nitrogen (N) in its purest form. Nitrogen is introduced into your tank in the form of nitrogen gas (N2) and ammonia/ammonium (NH3/NH4+). The ammonia/ammonium forms of nitrogen come from fish waste and are toxic to your fish. Luckily a nifty little nitrogen fixing bacteria called Nitrosomonas europaea loves to turn that ammonia/ammonium into nitrites (NO2-). These nitrites however remain relatively toxic to fish. The next step in the cycle is another type of bacteria called Nitrobacter sp. which turns nitrites into nitrates (NO3-) This form of nitrogen is also useable by plants and is only toxic in high concentrations. The final step, and what makes this a cycle, are a third type of anaerobic bacteria that turn nitrates back into the pure nitrogen forms like nitrogen gas (N2), and live plants which also breakdown nitrates.

Cycling your tank refers to getting this process started, adding the bacteria to carry it through, and waiting until those colonies of bacteria establish themselves in a high enough number to instantly convert any toxic ammonia/ammonium entering your tank into nitrites then nitrates, then back to nitrogen. For a more in depth description of the nitrogen cycle, download the Aquanomicon found on the right margin of The Fish Tank Biologist Blog Page.

Why bother cycling the tank though? Won't this process happen naturally? Yes, it will, but cycling before hand is necessary to prevent what we call a crash. There is a delicate balance between the amount of ammonia/ammonium produced from fish waste and the amount of bacteria present to convert it. If we add too many fish or don't cycle properly, the ammonia skyrockets and fish die from ammonia poisoning, causing a crash. The whole point of cycling is to establish this bacterial ecosystem so our fish are buffered from any toxic spikes. Ammonia poisoning is a painful way to die, and as responsible hobbyists, we must avoid it at all costs. And yes, the general scientific consensus is that fish do feel pain (Culum, B. 2016).

Which Method to Choose?

Now that we know why we must cycle our fish tanks, it's time to choose a method. The first step is to decide how you are going to cycle your aquarium. The process starts by adding ammonia/ammonium to your tank, which is converted to nitrites then nitrates as described above. This can be accomplished one of three ways. The first is by adding pure liquid ammonia to the tank. The second method is to add organic material which will break down into ammonia/ammonium. These are methods of fishless cycling. Adding liquid ammonia is the most humane and efficient, and the method I recommend. The drawback is having to handle pure liquid ammonia, which is perfectly safe if handled correctly, but should be avoided if small children are involved in the process. Adding organic material takes a little longer and leaves more waste and debris in the tank. However, you don't have to meticulously measure out toxic chemicals. The third option is add ammonia/ammonium in the form of fish waste. By adding fish to the tank, they begin to produce waste and that waste breaks down into ammonia/ammonium. The drawback here is, as mentioned above, an ammonia spike could cause ammonia poisoning and the death of your new fish. I would recommend fishless cycling. First though, we need a source of nitrifying bacteria.

Bacteria Seeding

Before you begin adding ammonia/ammonium to your tank you'll want to secure a source of nitrifying bacteria. The nitrifying bacteria is the N. europaea and Nitrobacter sp. species mentioned above. The source is any form of filter media, substrate, decoration, plant, or water sample from an established aquarium. It's best to use filter media, specifically biological filter media if you can, as this source will house the largest amount of nitrifying bacteria in the least amount of space. Biological filter media usually takes the form of ceramic or a porous plastic and is usually kept inside one of the filter compartments. If you can't get ahold of any filter media or a water sample from an established tank, don't worry, your local fish store or an online vendor should have a bottle of nitrifying bacteria available for purchase. I tend to avoid these however as the survival rate of living bacteria kept in the EDTA solution in the bottles is much lower than a fresh sample from an established tank. If you can't get access to either a fresh sample of filter media or a bottle of bacteria, it's okay. While it's not ideal, and it takes much longer, there is N. europaea and Nitrobacter sp. floating around in the air. They will eventually find their way into your tank and as long as there is ammonia/ammonium present, they will begin fixing the nitrogen.

Before you rush out to grab a scoop of gravel or a piece of filter media from your buddy's tank, consider this one drawback. While this source has the highest number of living bacteria. it also may have other microorganisms that live in a tank as well, including parasites like ich. However, if you fishless cycle, this won't matter as the parasites will die off from lack of hosts before you add fish. Ideally, you would seed bacteria with a source from an established tank then fishless cycle. If you cycle with fish, maybe consider using the bottled bacteria.

Fishless Cycling

The concept of fishless cycling is relatively straightforward. We want to complete the nitrogen cycle before we add fish to the tank. This means adding a source of ammonia for our sample of nitrifying bacteria to start fixing. Your options are to either add pure liquid ammonia to the tank, or a source of organic material that will break down into ammonia/ammonium. With both methods, it is crucial to complete the cycling process before you add fish, invertebrates, or coral as the concentrations of ammonia/ammonium will become toxic to animals.

Cycling with Pure Ammonia

When I was in high school I conducted an experiment in which I sterilized several five gallon tanks with an ultraviolet light, then seeded the tanks with a live culture of N. europaea I procured from a lab in Michigan. The first tank had an ammonia concentration of 2ppm, the second 4ppm, the third 8ppm, and the fourth 16ppm. The point of my experiment was to find out what the concentration of ammonia was to grow the bacteria culture the fastest, and if there was a concentration high enough to kill off the bacteria. I found the bacteria grew at the fastest rate at 8ppm, and did not die off and remained viable even at 16ppm.

With this in mind, we'll shoot for a concentration of 8ppm of ammonia after we seed our tank. You'll need a couple items to begin fishless cycling with ammonia. The first thing is an ammonia, nitrite and nitrate test kit. I recommend the API test kits as they are easy to perform and the ammonia test goes up to 8ppm. The second item is a plastic syringe that measures down to a tenth of a milliliter. Don't use a pipette as it is too easy to accidently spill ammonia from it. The syringe uses negative pressure to keep the liquid safely inside. Finally, you'll need pure liquid ammonia. You can purchase ammonia chloride from a couple different aquarium product companies. These work well and are in smaller amounts than the larger containers of pure cleaning ammonia. The economical option is to purchase cleaning ammonia with no additives from your local hardware or superstore. Make sure the ammonia you buy has no added chemicals, soaps, or detergents.

Now simply add your bacteria source and start adding ammonia to maintain 8ppm daily. To reach 8ppm you'll want to add 0.008 milliliters per liter, which is 0.03 milliliters per gallon. Multiply 0.03 by your number of gallons to determine how many milliliters of ammonia to add for your initial dose. For example, I have a 38 gallon tank times 0.o3 equals 1.14 milliliters of ammonia. Thereafter, the amount of ammonia you add will depend on how quickly the nitrifying bacteria fixes it.

After a day or two it's time to start measuring nitrite. As your nitrite levels begin to rise from 0.5 to 1 to 4ppm allow your ammonia levels to drop to 2ppm. After a week, measure your nitrates. As the nitrate levels begin to rise from 1 to 5 to 20ppm, keep decreasing your ammonia to .25 ppm. Eventually, you will see that the ammonia you added only a day before will have disappeared within 24 hours, your nitrites will also read 0ppm and nitrates will continue to rise (if you don't have live plants). When you read 0ppm ammonia, 0ppm nitrites and above 5ppm nitrates, your tank is cycled. Now you can perform a 25% water change to lower your nitrates to below 20 ppm and start adding fish. For a more in depth description of this process, download the Aquanomicon on the Fish Tank Biologist Blog Page.

Cycling your tank with organic material is the exact same process, except you will add fish food to the tank to decompose into ammonia/ammonium. I don't recommend this process as you can't control the amount of ammonia added and the fish food creates entirely too much waste and debris that has to be removed manually or filtered out. It's more hassle than its worth honestly.


Every tank is different. The number one question I get is, why hasn't my tank cycled yet? Some tanks cycle quickly, others can take months. The key is to keep testing your ammonia, nitrites and nitrates. As long as you see these numbers heading in the right direction your nitrifying bacteria is working and will reach the required population size soon. This article covers fishless cycling, mainly with ammonia. I'll be writing another article in the future regarding cycling with fish. I don't recommend this process, but if it's all you can do, it's important to do it right.

  • Culum, B. (2016) Fish pain: an inconvenient truth. Animal Sentience 3:32.

  • Scott Blake



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