Fish swimming in a tank with some water plants

How to Cycle a Fish Tank

Whether you’ve just gotten yourself a vibrant betta fish or some other fish, you need to cycle the fish tank to keep the water safe and healthy.

What Does Cycling a Fish Tank Mean?

This might be the first time you’re hearing about the concept of cycling a fish tank. It simply refers to filtering the water to remove the toxins dispelled by the fish. The process involves encouraging the growth of bacteria, which will digest excreted ammonia and turn into nitrite. Since these nitrites are toxic, the next step is to filter them in order to convert them into nitrates, which are not harmful for your fish. Even so, these nitrates will kill your fish’s appetite so you will still need to clean the water out at regular intervals.

How to Cycle a Fish Tank

This method is simple and suitable for beginners.

Step 1: Ammonia Production

Your tank won’t have any fish at this point so you will have to find an alternative way to introduce ammonia in the water. Add fish food flakes to the tank every 12 hours. Over time, they will decay and release ammonia.

Get the appropriate testing equipment from a local pet store. Check the ammonia levels every few days to ensure that the ammonia level reaches 3 ppm (parts per million). If you’re not there yet, add more of the flakes and wait. Keep this up for around 7 days.

Step 2: Nitrite Test

Use the right tool from your test kit to check the nitrite level in the tank. Once your kit picks up the presence of nitrites, the nitrogen cycle of your tank has started. Add more ammonia.

Step 3: Nitrate Test

After a few weeks, you’ll notice a drop in nitrite levels. Once this starts happening, start testing for nitrates. Once your ammonia and nitrite levels are zero, your cycle is complete.

Step 4: Bringing Your Fish Home

Gradually start adding your fish. Add a few and wait a few days before adding more.

Performing the Cycle with Fish in the Tank

This is an emergency method of cycling your fish tank. If you don’t have anywhere to keep your fish and you got them without cycling your tank, you’ll have to resort to this. Just remember that some of your fish may die in the process.

Step 1: Introduce Resilient Fish to Your Tank

Add fish which excrete more waste. They should also have a strong immune system to survive the excess levels of ammonia and nitrites until the water starts encouraging the growth of beneficial bacteria.

Don’t add more than 1 or 2 fish for every 10 gallons. Too many fish will increase the ammonia content and kill your fish in the early stages. Some contenders include Minnows, Guppies, Tetras, White Clouds and so on.

Step 2: Feed Your Fish

Provide decent sized meals to your fish every two days. Overfeeding will increase the toxicity of the ammonia before the beneficial bacteria has had the time to grow.

Step 3: Changing the Water

Change the water around once a day to avoid excess build-up of toxins from the ammonia. Don’t clean it too much or you’ll get rid of too much of the ammonia and nitrite and the bacteria won’t have anything to feed off and grow.

Add a de-chlorinator if necessary to keep the water free from chlorine.

Step 4: Checking the Water

This step is the same as the testing section of the fish-free cycling process. Once the ammonia and nitrite levels are at zero, the water is safe for your fish. You can add more fish now if you like. Don’t add too many or you risk increasing the ammonia levels again and the cycle will have to be repeated.

Shortcuts for Cycling Your Fish Tank

If you don’t have the time or are unsure of how to complete the entire cycling process, there are some alternate routes:

  • Use a media filter from an already cycled tank to cleanse and nourish the water.
  • Get some gravel from a filtered tank and put it in a mesh bag. Suspend it in your tank and the cycle will start on its own.
  • Add live plants to your tank. Not only will they kill the toxins, but they’ll also release nutrients that can help your fish thrive.


Don’t use items from a cycled tank if you are aware of any harmful bacteria present in it. The transfer of these to the new tank will almost certainly kill your fish before the cycle even begins.

Troubleshoot for Failed Cycling Attempts

If you’ve cycled your fish tank and you still notice the ammonia levels rising or your fish are dying, here is what could have potentially gone wrong:

  • If you’re cycling with the fish in the water, chances are the fish are not strong enough to withstand the high levels of ammonia. If your fish look tired, aren’t eating or look like they’re in pain, the ammonia levels are too high and the water in the tank needs to be changed. Again, it is advisable to perform a fish-less cycle in your tank.
  • If it’s been a few days and things aren’t moving forward, either your test kit isn’t working or there isn’t enough ammonia in the water. You’ve either added very little fish food or you’re cleaning your tank out too much.
  • Chlorinated water prevents the cycling from happening. Check for the presence of chlorine and make use of a de-chlorinator before starting the cycling process.
  • Cycling often encourages the growth of algae. Turn the lights in the aquarium for a few hours to stunt the growth of algae. You can also try and inject carbon dioxide in the water to prevent algae bloom.

The Cycle That Never Ends

The nitrogen cycle is necessary to help your fish thrive. It’s necessary to complete the cycle before getting new fish, but even after you’re done, you’ll need to keep an eye on the ammonia, nitrate and nitrite levels in the tank to ensure that everything is in balance and that your fish are happy.

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