Jennifer O. Reynolds, M.A. & Nora Hickey, D.V.M.
Whether you’re setting up your first aquarium or expanding your collection, cycling your aquarium is essential. Discover tips and tricks for three effective methods to kickstart the nitrogen cycle: using old filter media, fishless cycling, and cycling with fish.

Cycling your new aquarium is one of the most important steps to prepare it for your new fish. “Cycling” a fish tank means establishing a sufficient population of beneficial bacteria in the aquarium’s biofilter that will handle the waste that your fish produces. If your aquarium does not have enough of these bacteria prior to adding fish, the waste your fish produce will build up in the aquarium and poison them.
The nitrogen cycle in an aquarium starts with feeding your fish. Fish excrete ammonia through their gills into the aquarium water as a waste product from the proteins they consume. The beneficial bacteria in the biofilter convert the ammonia into nitrite and finally nitrate; both ammonia and nitrite are very toxic to fish. High ammonia levels can cause gill damage, neurological problems, and death. High nitrite levels cause death and piping (gasping at the surface of the water).
There are three different ways to cycle your aquarium to ensure you have a sufficient population of beneficial bacteria in your biofilter to convert waste from new fish you add.
Using Old Filter Media
Our favourite method of cycling a new aquarium is adding beneficial bacteria from the filter of an existing, healthy aquarium. If you do not already have an aquarium, consider whether you may have a friend, helpful local fish store staff, or member of a local hobbyist group who can provide you with a dirty piece of floss or sponge from their established healthy aquarium filter, or if you can squeeze out their sponge filter into a container. You can then bring this lovely, living sludge—loaded with beneficial nitrifying bacteria—back home and immediately introduce it to your filter and tank.
Adding filter sludge might seem counterintuitive to a new hobbyist, but aquariums are tiny ecosystems that are not meant to be kept perfectly clean. This is the best method to seed your new aquarium with nitrifying bacteria and prevent problems such as ammonia and nitrite toxicity that commonly occur in new aquariums. You can introduce a few fish shortly after you add the bacteria, then slowly expand your fish population as the bacteria multiply to support the fish biomass. Since fish food is the main source of ammonia in an aquarium, it is wise to feed sparingly during the cycling process.

Fishless Cycling
If you can’t obtain bacteria from an established aquarium, you can consider undertaking a “fishless cycle”, which is when you add an ammonia source to your aquarium to start the growth of nitrifying bacteria before adding live fish. Although this requires patience because it takes several weeks, you will not be putting any fish at risk, so it can be a lower-stress way for new fishkeepers to get started. To begin a fishless cycle, add small pinches of fish food every few days over a period of several weeks, to mimic the waste production that the living fish will create when they are added. This can help to start, feed, and grow nitrifying bacteria colonies prior to the introduction of your fish.
The expected progression of a fishless cycle begins with the presence of ammonia. As nitrifying bacteria establish themselves, ammonia levels decrease while nitrite levels rise. As the cycle continues, nitrite levels decline and the end-product of the nitrogen cycle, nitrate, accumulates. Nitrate is then managed primarily by ongoing aquarium maintenance including water changes. When bacteria are properly established, the addition of ammonia sources such as fish food will no longer result in measurable ammonia or nitrite; the bacteria will quickly convert these to nitrate. However, the bacteria require this ongoing source of ammonia to stay alive, so be sure to continue adding some food or other ammonia sources to maintain your nitrifying bacteria, until you add your new fish and they can take over eating the food and producing waste to maintain the bacteria.
It is also possible to directly dose the aquarium with ammonia to jump-start the cycle and maintain nitrifying bacteria colonies, but this requires careful selection and dosing of an appropriate ammonia source as well as monitoring of ammonia, nitrite and nitrate over several weeks or more. Using ammonia for a fishless cycle is challenging and is not necessary to cycle home aquariums.
Cycling with Fish
Another option is to cycle your aquarium by adding two to three small, hardy fish for a “fish-in cycle”. It is important that the aquarium temperature has been stable for a few days and your aquarium is otherwise set up. New aquariums are sensitive to new fish additions and cannot handle them the way that a mature aquarium can, so it is critical to slowly build your fish population up over weeks or months rather than add them all at once.

Popular and relatively hardy species for beginners to use with fish-in cycling include zebra danios, platys and cherry barbs. Cycling your aquarium with fish in it also requires routine monitoring of ammonia and nitrite levels, frequent water changes, very light feeding, and potentially, prevention and treatment of ammonia and nitrite toxicity if these levels approach toxic levels during cycling.
If ammonia levels approach an unsafe level in an aquarium during cycling (> 1 ppm TAN is a good benchmark), perform large water changes (at least 50%) to remove the ammonia. Any level of nitrite detectable by an over-the-counter water quality testing kit is toxic; nitrite toxicity can be prevented by adding aquarium salt to the water (see our conversation about this here). Products such as Seachem Prime can detoxify ammonia and nitrite and are often useful to use when cycling a tank with fish.
Fish-in cycles are popular, as new fishkeepers are excited to get to the fishkeeping part sooner rather than later, but it can also lead to disappointment if it is not carefully managed. No one wants their first experience with their new aquarium to end in the death of their new fish because of ammonia or nitrite poisoning.
Conclusion
Our favorite method for cycling a fish tank is to get filter media from a friend, but there are other options for cycling your aquarium if you don’t have easy access to old filter media or if new fish came into your life unexpectedly and you are now trying to figure out how to get their tank cycled. Whatever method you use, closely monitor water parameters when fish are added to the tank and take corrective actions if there are problems (usually by performing large volume water changes). If you do this, you should be able to successfully cycle your aquarium and have happy and healthy new fish.