When you get started in the aquarium hobby, you will hear a lot about the “cycling” stage that occurs in the beginning. You might read how you want to use hardier fish to “cycle” your aquarium, or how it takes about a month for the “cycling” process to complete. This “cycling” is referring to the Nitrogen Cycle that occurs in every new aquarium. Here are some popular questions and answers about what it means when an aquarium is “cycling,” as well as what to expect during this time.

What Does “Cycling the Aquarium” Mean?

When you start a new aquarium, you need to establish colonies of beneficial bacteria, like Nitrosomonas and Nitrobacter bacteria, to convert both ammonia and nitrites into nitrates before they can poison your fish. The process to do this is called “cycling” or “conditioning” your aquarium. It is also called the Nitrogen Cycle.

Natural toxins immediately begin building in your aquarium water. The helpful bacteria that exists in the aquarium water begins the Nitrogen Cyle, the process of converting these toxins into less harmful substances. Lethal ammonia is converted into less harmful nitrites and then into nitrates. Using tools like aquarium filters harnesses the work that beneficial bacteria does in your aquarium.

Your aquarium has been successfully cycled when the populations of the helpful bacteria are balanced with the waste load of your aquarium. This keeps the ammonia and nitrite levels at zero.

How Does Ammonia Get in the Aquarium?

Ammonia is introduced into your aquarium from fish’s gills (respiration), fish wastes (solid waste and urine), and biological decomposition of organic materials (plants or dead fish). When too much ammonia accumulates in your aquarium, it can kill fish, plants, and other living organisms. Ammonia levels tend to peak around day 10 of the cycle. That is why the Nitrogen Cycle is so important – the beginning process converts ammonia to less harmful substances.

How Do Nitrites Get in the Aquarium?

There is a naturally occurring species of bacteria, Nitrosomonas, that feeds on ammonia. This Nitrosomonas bacteria converts ammonia into nitrites. Ammonia can be extremely toxic to fish. While Nitrites are less toxic to fish than ammonia, they will still kill fish. Nitrite levels start to peak around day 16 of the cycle. Nitrites have to be kept at a low level in order to keep your fish alive. That’s where the second step of the Nitrogen Cycle comes in.

How Do Nitrates Get in the Aquarium?

There is a naturally occurring species of bacteria, Nitrobacter, that feeds on nitrites. This Nitrobacter bacteria converts nitrites into nitrates, which is a much less harmful chemical compound. Nitrates are a food source for live aquarium plants, so live plants help keep the nitrate levels low. Algae and some bacteria also use nitrates as food. If you do not have a planted aquarium, routine monthly partial water changes keep nitrates at a level that is not stressful to your fish.

How Do Beneficial Bacteria Get in the Aquarium?

Ironically, adding fish to your aquarium helps introduce small quantities of beneficial bacteria, both Nitrosomonas and Nitrobacter. Some of the bacteria is on the fish, while some of it is in the water that your fish were in.

There are also bacterial additives you can use to help generate beneficial bacteria more quickly and cycle your aquarium more quickly.

What Do Beneficial Bacteria Need to Survive?

Beneficial bacteria need two simple things to thrive: food and oxygen. Food is provided via fish respiration and fish waste. Oxygen is provided via the dissolved oxygen in the water – the bacteria just utilizes what is already there. That is one reason aquarium circulation is so important, as the water movement helps distribute oxygen evenly throughout the aquarium.

How Long Does the Cycle Take?

It takes about four weeks to cycle an aquarium at home. Under ideal circumstances, it may take approximately 23 days for the Nitrogen Cycle to complete. However, it is always better to be safe than sorry, so you should give the cycling period a full 28 days.

If you wonder why you are advised to start your aquarium with some “starter” fish and then add other fish a month later, it is due to the length of the Nitrogen Cycle. Some fish are much more hardy than others. Hardy fish include Mollies, Guppies, or Tetras. They handle the stress of cycling the aquarium much better than other fish, like Discus. Once your Nitrogen Cycle is over, you can add less hardy fish – if your water parameters are correct.

How Can You Tell the Cycle is Complete?

The chemical compounds being discussed (ammonia, nitrites, and nitrates) are invisible to the human eye. The only way to know if your aquarium is finished cycling and safe to add more fish is to test the water. This can be accomplished using aquarium test kits. You can find test kits made for freshwater aquariums, marine aquariums, and reefs. There are lots of water parameters you can test for, but you only need to measure ammonia, nitrites, and nitrates in regards to your aquarium cycle.

When the ammonia and nitrite levels are at a zero level, and the nitrates is below 50 ppm, your aquarium will have completed your Nitrogen Cycle. You want the ammonia and nitrites at zero, since both compounds are toxic to fish. Nitrates above 50 ppm start to put stress on your fish. You can keep nitrates at a manageable level by performing monthly partial water changes of 20% to keep nitrates in check.

How Can You Tell if Your Nitrogen Cycle is Not Occurring?

You can tell your aquarium is not cycling if your fish develop New Tank Syndrome. New Tank Syndrome occurs when ammonia or nitrite is not properly converted to nitrates, so your aquarium water becomes toxic to your fish. This primarily happens when too many fish are added to a new tank and the beneficial bacteria population cannot grow fast enough to keep up with demands.

You can prevent New Tank Syndrome by using only a few starter fish for the cycling period.

What Are Symptoms of New Tank Syndrome?

There are a few distinctive characteristics that define New Tank Syndrome. If your fish lose their color, lie at the bottom of the aquarium, or hide in the corners of the aquarium with clamped fins, they are most likely suffering from New Tank Syndrome. Since this occurs when ammonia or nitrite is not being converted to nitrates, you can immediately help your fish by doing a 25% partial water change. This partial water change will lower the levels of ammonia or nitrite to a less toxic level in an attempt to save the fish. If you have a test kit, you can test to see if you lowered the ammonia or nitrite levels enough.

After your 25% partial water change, you will want to perform 10% partial water changes once a week until the end of your cycling period. This will keep the ammonia or nitrite level at a less toxic one, while giving the beneficial bacteria a chance to grow. These 10% partial water changes may cause your aquarium to cycle for longer than 4 weeks, but they will help to keep your fish alive. You do not want to do any water changes that are greater than 10% of your aquarium, as you could cause more harm than good by taking out the beneficial bacteria.

Why Has a Hazy Cloud Developed in the Aquarium Water?

If a hazy, gray cloud develops in your aquarium water a few days after you add a few starter fish, don’t panic. It is just the beneficial bacteria “floating” through the water before settling on surfaces throughout the aquarium. This gray haze is typically called a bacterial bloom, and it is a period when the beneficial bacteria begins to multiply and establish themselves in their new home. The cloud will dissipate on its own in a few days to a week, as the bacteria starts to settle.

What Causes Cycling to Take Longer Than 4 Weeks?

Typically, your aquarium will take longer than 4 weeks to cycle if the waste level in the aquarium is building faster than the beneficial bacteria population. When elevated ammonia or nitrite levels occur, it causes your aquarium to get stuck mid-cycle.

Some of the reasons your aquarium could get stuck mid-cycle include:

  • too many fish were added at the beginning of the cycle
  • new fish are being added too soon (before the initial cycle is over)
  • fish are being over-fed, so higher waste levels are generated (either through fish waste or through uneaten food rotting at the bottom of the aquarium)

You will need to reduce the waste levels through partial water changes, aquarium cleaning, or removal of some fish in order for the bacteria growth to catch up and allow the aquarium to finish cycling.

Can I Add More Fish After the Nitrogen Cycle Ends?

After the initial cycling period, it is safe to add more fish to your aquarium. However, you need to remember that every time you add fish to your aquarium, the beneficial bacteria population must grow to compensate for the increased bio-load. In essense, you are putting your fish through mini-cycles every time you add new fish – it is just not as intense as the cycling that occurs during the initial set up.

When you add new fish to your aquarium after the initial cycle, just add a few at a time. If you add more than your beneficial bacteria can handle, the ammonia or nitrite levels will build up too quickly, and your fish could develop New Tank Syndrome. Even a well established aquarium has to be careful when adding new fish so as not to develop New Tank Syndrome.


For additional information about starting your aquarium cycle, check out Fishkeeping 101: Cycling the Aquarium.

About The Author Kristen Sydelko

Kristen is the Web Coordinator at PetSolutions. She has over 5 years of experience working in the pet care industry, with many more years of pet ownership experience! When not at PetSolutions, Kristen enjoys spending time with her family (which includes an extremely spoiled Lab mix), crafting, and trying to decide when to set her fish tank back up.

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