How to Cycle a Fish Tank

Posted by
Cameron Wise on December 16, 2022

By far the most tedious aspect of owning an aquarium is the initial setup. This involves setting up the pumps, filters, and decorations before adding appropriately filtered water. Once the tank’s components are put in place, the longest of setup processes, cycling, is ready to begin. Aquarium cycling is often overlooked by beginners who wish to see their tank alive, but it is a crucial step in creating an environment for happy, long-living fish. Below you can find information on what cycling is, why it is important, and how to cycle your aquarium.

What is aquarium cycling?

Aquarium cycling is the process of creating a biologically safe environment for fish in a new tank. It involves introducing nitrifying bacteria into the aquarium to regulate the nitrogen cycle. These bacteria curb the effects of ammonia buildup caused by the breaking down of fish waste. To complete the nitrogen cycle, ammonia must be converted to nitrite, and the nitrite must be converted to nitrate.

Why do aquariums need to be cycled?

Before fish are introduced into the environment, nitrifying bacteria must spread themselves across the aquarium’s biological filters to handle the volume of ammonia produced by the fish. If fish are introduced to the tank too quickly or if too many fish are introduced at once, the biofilters will not be able to keep up with the amount of ammonia produced by the fish. Even after cycling an aquarium properly, it is best to introduce your fish one at a time to ensure the tank is biologically prepared for more fish.

If you do not cycle your aquarium, the nitrogen cycle will still occur, but your fish will likely be harmed in the process. The effects of built-up toxins in a new aquarium contribute to what is known as “New Tank Syndrome.” Species of fish that are not hardy are more prone to symptoms of ammonia exposure and require a cycled aquarium to live healthy lives.

How long does an aquarium take to cycle?

Aquariums generally take four to eight weeks to cycle. Factors such as the size of the tank and the water’s pH and temperature can affect how long cycling takes. To prevent the cycling process from taking longer than necessary, closely monitor the temperature and pH of your aquarium’s water. The pH should remain between 7.0 and 7.8, while the temperature should maintain a range of 83°F to 87°F. Not only does this expedite the tank’s cycling, but it will also ensure the stability of the environment the fish will live in. You should also test the tank’s ammonia, nitrite, and nitrate levels every one to two days when cycling your aquarium. If the aquarium is cycling properly, you will first notice a drop in ammonia and a significant spike in nitrites. Once the nitrite levels are high enough, nitrate-producing bacteria will begin populating the tank. Once these bacteria populate the tank, nitrite levels will fall. Once the ammonia and nitrite levels reach zero and nitrates are being produced, the cycle is complete.

coral fish tank

What do I need to cycle an aquarium?

Before cycling an aquarium, ensure you have the following items available:

An aquarium cannot be cycled until it is equipped with all of the components it requires to house the fish. Certain water contaminants, such as chlorine and chloramine, affect the nitrogen cycle and the water’s PH, so a quality water filter is necessary when filling your tank.

Learn more: Why you should use reverse osmosis water for your aquarium

How to cycle an aquarium

When cycling your aquarium, you have the option to cycle without fish, with fish, or with plants in the tank. It is highly recommended that you cycle your tank without fish, especially if you are a beginner. Fish in the tank during this process are exposed to higher-than-normal levels of ammonia, and even hardy fish can suffer as a result. Plants, on the other hand, are unpredictable and can lead to frustration when cycling a tank.

Cycling without fish

Cycling an aquarium without fish is more time-consuming than cycling with fish, but it is also more humane. Consequently, it is the more popular way to cycle an aquarium. Note that you should introduce fish with this method once your aquarium shows signs that it has cycled.

1.  Set up the tank’s components

Before cycling the tank, you must ensure that the pumps, filters, substrate, and other tank components are set up and functioning properly. While the aquarium is cycling, all bubblers, heaters, pumps, and filters should be kept running. This allows nitrifying bacteria to accumulate on the surfaces of the aquarium, particularly the filters and substrate. The bacteria that cling to these surfaces reproduce and maintain the nitrogen cycle when fish are added to the tank.

Learn more: What is an aquarium air pump? | What is an aquarium water pump?

2.  Check the water’s pH

Maintaining a pH of 7.0-7.8 is ideal for the nitrogen cycle to occur the most efficiently. It is also the perfect pH for most freshwater fish. Water that is too acidic (pH of less than 7.0) is particularly detrimental to the nitrogen cycle because it allows ammonia levels to rise when it is not wanted. Chlorine, a chemical present in tap water, drops the pH of water below desired levels. In contrast, too high of a pH is also detrimental to the nitrogen cycle. When the pH rises above 9.0, ammonium converts into ammonia, disrupting the tank’s environment. Throughout the whole process of cycling and after fish are added to the aquarium, it is important to check and maintain the water’s pH to preserve the health of your fish. The pH of water can be tested with pH test strips or an all-in-one test kit.

3.  Add ammonia to the tank

To begin the nitrogen cycle, ammonia must be present in the tank. A simple and effective way to raise ammonia levels in an aquarium is by adding fish food. You should add the amount of food that you would if fish were present in the tank. If you plan on incorporating six fish in your tank, for example, you should add an appropriate amount of food for six fish. When the food decays, it releases ammonia into the water, kickstarting the nitrogen cycle.

4.  Check ammonia levels

Once the food decays, check the ammonia levels of the water. The ideal ammonia concentration in a tank is about 3 parts per million (ppm). Ammonia levels as high as 5 ppm are acceptable, but fewer than 3 ppm of ammonia may fail to produce enough nitrite to properly cycle your tank. If the ammonia levels are too low, add more fish food, allow it to decay, and check the levels once more. If the ammonia levels are too high, change out about 10 to 20 percent of the tank’s water with treated water.

Check the ammonia daily for one week and add food if the ammonia drops below 3 ppm. Likewise, perform a water change if the ammonia rises above 5 ppm. Ammonia, alongside pH, nitrites, and nitrates, can be checked with an aquarium test kit.

5.  Check nitrite levels

Once the water has maintained appropriate ammonia levels for a week, the nitrite levels in the tank should be detectable. If nitrites are not detectable yet, allow a little more time and continue to monitor the ammonia levels and pH. Once nitrite levels begin to rise, they will spike quickly as there is nothing to regulate them yet. The nitrites feed on ammonia, so you must maintain ammonia levels during this stage as well. If the ammonia runs out, the nitrates can disappear, and you will need to start the cycle over again. Once the nitrite levels begin to drop while the ammonia levels are suitable, you can move on to the next step.

6.  Check nitrate levels

When nitrite levels decline in a tank, that is a sign that either nitrifying bacteria are feeding on the nitrite or that ammonia levels are too low. If nitrates are detectable in your tank, that is a sign that the nitrite levels are declining due to the presence of bacteria. Once nitrate levels are detectable, it is the beginning of the end of the cycle. To ensure the bacteria have enough to feed on, add half as much fish food as you did on the first day about once every two to three days. You should notice that the ammonia and nitrite levels continue to decrease. Once the ammonia and nitrite levels reach zero and the nitrate levels remain, the tank has finished cycling.

7.  Introduce fish into the tank

Once the tank has finished cycling, incorporate fish into the tank slowly. To begin, add one or two fish so the bacteria do not become overwhelmed. After a day or two, check the ammonia levels in the tank. If they remain at zero, you can incorporate more fish into the aquarium. Continuing to monitor and regulate the ammonia, nitrite, and pH levels of the aquarium is essential in maintaining the health of your fish.

Learn more: How to maintain a freshwater aquarium

Cycling with fish

Cycling a tank with fish is similar to cycling a tank without fish, but the source of ammonia differs. Rather than adding food to the water, the fish provide ammonia in the form of waste. In addition, you will need to perform frequent water changes with fish present. Fish-in cycling is not recommended in most situations, but it can be effective if you populate your tank with hardy fish.

two fish in tank

1.  Set up the tank’s components

This step is identical to step one of fishless cycling. Before you add fish to your tank, ensure all components are installed properly and are in working order. This includes all pumps, filters, bubblers, temperature regulators, and all other components. This creates a suitable environment for your fish to live in.

2.  Add a few hardy fish to the tank

Hardy fish are less affected by ammonia and other contaminants than most fish species, so they are the only suitable type of fish to be in the tank while cycling. Do not add more than three fish to the tank. If too many fish live in the tank during the cycle, they will produce too much waste and cause ammonia levels to spike too high.

3.  Feed the fish

Once the fish have been added, you will want to feed them sparingly. This can mean feeding them less frequently or feeding them smaller amounts at a normal frequency. Avoid overfeeding the fish, as this will cause them to produce more waste and lead to the additional production of ammonia via decayed food. Once the cycle kicks off, you can begin feeding the fish regular amounts of food.

4.  Perform water changes

Change about 10 to 20 percent of the tank’s water twice every week. This helps regulate ammonia, nitrite, and other contaminants’ levels. Water changes of this frequency should continue as long as your aquarium operates.

5.  Monitor contamination levels

While making water changes, you will need to constantly monitor the tank's ammonia, nitrite, and nitrate levels. Once the nitrate levels remain consistent and the ammonia and nitrite levels are undetectable, the cycle has been completed. Additional fish can then be added to the tank.

Cycling with plants

Aquarium cycling with plants is far less predictable of a method than fishless and fish-in cycling. Plant death and algae growth can cause the cycle to fail and require you to start the process over. As a result, this method is not recommended over fishless cycling, but it is more humane than fish-in cycling. Before cycling a tank with plants, you will need to purchase nitrogen-containing fertilizers.

planted aquarium

1.  Setup the components of the aquarium

Like step one in the previous methods, ensure that all pumps, bubblers, filters, heaters, and other components are added to the aquarium.

2.  Disinfect and add plants to the aquarium

Before adding plants to the aquarium, disinfect them to reduce the risk of outside bacteria and parasites from harming the tank’s environment. Hardy plants are the best plant type to add to an aquarium while cycling. Water column feeders are ideal because they absorb ammonia and nitrate better than other plant types.

3.  Incorporate aquarium lighting

The lighting of a planted aquarium is essential in allow the plant to photosynthesize. Ensure that the lighting setup you use is optimal for your plants both while cycling and after. At this stage, plants should receive about ten to twelve hours of light per day.

4.  Add fertilizer

Now that the lighting is set up, you can add fertilizer to the tank. If you are using water column feeders, liquid fertilizer is the best option. For root plants, place root tabs directly under the plants’ roots.

5.  Perform water changes

When cycling with plants, you should change about 30 percent of the tank’s water each week. This reduces algae growth, especially in the first few weeks after introducing plants to the water.

6.  Watch for new growth

If you see new growth in your tank, that is a sign that the tank is completing its cycle. You may notice some algae growth along the tank walls and substrate. In this phase, some algae growth is acceptable because it signals there are enough nutrients for the plants to thrive. If your tank has an abundance of algae, lower the lighting duration to about eight to ten hours a day.

7.  Monitoring the water contamination levels

Throughout this process, you will need to monitor the ammonia, nitrite, and nitrate levels of the aquarium’s water. To test if your tank is cycled, add around one ppm of ammonia to the tank. If ammonia levels are undetectable after 24 hours, nitrites are undetectable, and nitrates are present, the tank has completed its cycle.

Can I cycle an aquarium in 24 hours?

Adding bacteria manually can help a tank cycle quickly, but it can lead to problems for your fish later on. If you choose to speed up the hydrogen cycle in your tank, you will need a filter and substrate from an already cycled tank to produce the most desirable results. While bottled bacteria and recycled filters and substrate speed up the nitrogen cycle, the best option for your aquarium is to let the process carry out naturally.



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