How To Cycle A Fish Tank: The Complete Beginner’s Guide

Aquarium cycling is one of the most important steps in fish keeping.

The cycling process prepares your tank for your new fish by mimicking the natural biological and chemical processes that occur in wild aquatic habitats.

Whenever you set up a new tank you must complete a nitrogen cycle before you add your fish.

This process will not only ensure that your tank is ready to support life, but it will give you an up close and personal view of a natural process at work.

Do you have a new tank that is ready to be cycled?

Keep reading to learn how to safely complete a fish tank cycle and also special tips on speeding up the process.

What is the Nitrogen Cycle?

The Nitrogen Cycle

Nitrogen is the most common element in our atmosphere.

It is readily available not just on land but in aquatic environments too in the form of ammonia produced by waste and decay. The nitrogen in ammonia can be turned into nutrients that organisms can use. However, the nitrogen cannot be used by itself. It must be converted into a nitrate compound first.

The nitrogen cycle is the biological and chemical process that converts nitrogen from ammonia into nitrate compounds.

This process occurs both on land and in water.

In a functioning nitrogen cycle a beneficial bed of bacteria will build up naturally during the cycling process. This bacteria will convert ammonia, into nitrites, and then into nitrates.

Then when animals poop and plants die, the process begins all over again.

Nitrogen Cycle Steps and Timeline Explained

What is the nitrogen cycle

Step 1: Ammonia

It all starts with ammonia.

The first stage of nitrogen fixation is where bacteria in the substrate turn nitrogen (N2) into ammonia (NH3) by breaking down waste and decayed material.

This process can take 7 to 10 days.

The amount of ammonia in the water column will steadily increase as nitrogen fixation is taking place. Once there is enough ammonia available, the second step can begin.

Step 2: Nitrites

During the second stage a process called nitrification will take place.

Nitrite compounds (NO2) are produced by breaking down and adding oxygen to the available ammonia.

Nitrites are the bridge between ammonia and usable nutrients. They can be used by the bacteria in the sediment to form nutrient compounds. As nitrification continues the amount of ammonia will decrease as the amount of nitrites increases. Eventually, only nitrites will be present in the water.

This part of the process usually takes another 10 to 15 days, but the exact time will depend on water temperature and quality.

When the amount of available nitrites reaches its peak, the third and final step can begin.

Step 3: Nitrates

At this point more oxygen will be added to the available nitrites and it will turn them into nitrate compounds (NO3).

These nitrate compounds are the nutrients that are used by animal and plant life (you may be familiar with them in your garden fertilizers). As there is an increase in the amount of available nitrates, the amount of nitrites will eventually drop to zero.

After 15-20 days only nitrates will be present in the water. These nitrates are then available for the fish and invertebrates to use, but they cannot reach high levels which is why water changes are needed.

The entire process will be complete in approximately 6 to 8 weeks.

How To Cycle A Fish Tank

How to Cycle a Fish Tank

You should start your nitrogen cycle by filling your new tank with clean and de-chlorinated water.

The water temperature should be raised to above 70°F, but should not exceed 77°F. Also the pH should be 7.0 or lower (high temperatures combined with high pHs can cause ammonia to become toxic).

In natural aquatic environments the flow of water acts as a catalyst.

To mimic this in your tank you can equip your tank with a water circulation pump to simulate natural currents.

Next you need to fill the bottom of your tank with the substrate of your choice.

You can wait for the bacterial colonies to form in the sediment naturally or you can purchase bacterial cultures (cycling bacteria) from an aquarium supplier. If you have an old aquarium you can also import bacteria from the sediment or media filter. However, special care should be taken to make sure your sediment is clean and free of harmful bacteria.

Under gravel filters and hang on back filters are great for cycling tanks. An under gravel filter in particular can generate small currents that will help the process along.

Ammonia will not be available in your new tank. You will have to add it in yourself or allow it to occur naturally. You can use raw household ammonia from a hardware store, or you can rely on natural ammonia from decaying plants.

It is very important to keep water quality testing kits around during this procedure.

Your water quality, ammonia, and nitrogen content must be tested frequently to make sure that everything is going as planned. If you want to provide an extra source of ammonia then you can include plants in your tank, however only some aquarium plants are hardy enough to be in a cycling tank.

Java Moss is commonly used. Ferns, Waterweeds, and Amazon Swords are also commonly used because they are all hardy enough to be introduced to a cycling tank.

Follow these simple steps to cycle your fish tank:

  1. The first thing you need to do is fill your tank with clean and de-chlorinated water.
  2. Next heat the water temperature to above 70°F.
  3. Now add your substrate to the tank.
  4. Next you should power on your filters and gradually add trace amounts of ammonia to the tank.
  5. Test the water to make sure your ammonia content does not exceed 5 ppm.
  6. Turn on your water circulator and aerator and heat the water up to a maximum of 75°F.
  7. Add in your bacterial cultures or simply wait for natural colonies to appear (natural colonies can take up to 10 days to establish).
  8. Over the course of the next week or two, measure the ammonia and nitrite levels in the tank. You know that nitrification has begun when the ammonia content begins to drop and the nitrite levels increase.
  9. When the ammonia levels drop to zero and nitrites are present in large amounts, you can begin to test for nitrates. Check the nitrite and nitrate content of your tank over the course of the next 10 to 15 days.
  10. Once nitrite levels have dropped to zero and your test kits find nothing but nitrates, it is time to prepare your tank for fish. Change up to 50 percent of your tank’s water and clean your filters. Adjust the heat, pH, and other parameters to match the ideal tank conditions for your fish.
  11. You are now ready to add your new fish to your tank.

How To Speed Up The Cycling Time

Speed up the Cycling Time

It can take up to 2 months to cycle a fish tank for the first time.

Fortunately, there are plenty of ways to speed it up.

One of the best ways to reduce cycling time is to use pre-purchased bacterial cultures instead of waiting for them to appear. While it may take longer than a week for the colonies to appear naturally, ready-made colonies can establish in as little as 15 hours if the conditions are right.

If this is not your first tank you can also use the filter media or substrate from an existing tank that has already undergone the process.

New colonies of bacteria can be formed alongside your old colonies by carrying out the regular steps of the process. This combination of old and new colonies will make the procedure go even faster.

Also instead of relying on natural ammonia from plant decay you can purchase raw ammonia from a hardware store to add to your tank. This way you do not have to wait for your plants to rot first.

Another way to speed up your aquarium cycling is to increase the oxygen.

Nitrifying bacteria rely on an excess of oxygen to do what they do best. The more oxygen is available in your tank, the quicker the nitrification process will be. Keeping your tank well aerated will lead to a quicker procedure.

Air stones used alongside a good aerator can really make a difference in the amount of available oxygen.

Finally you can speed up the cycling process by simply raising your water temperature.

Bacterial colonies establish more readily in higher temperatures.

This option can be risky, and increases the possibility of ammonia toxicity. Be very careful not to raise the temperature above 77°F.

FAQs About Tank Cycling

How long does it take to cycle a fish tank?

It will typically take about 6 to 8 weeks to cycle a fish tank for the first time.

However the exact length of time will depend on the method used and if you have already established bacterial colonies.

How can I cycle a fish tank in one day?

Despite what you may have read, unfortunately it is not possible to cycle your tank in one day.

Ammonia, nitrite, and nitrate are the three major chemicals involved in this process.

Increasing the amount of any one of these will lead to a quicker process however it will still take at least 7 days.

Can you cycle a tank with fish in?

Some fish keepers will suggest adding one or two fish to your tank whilst it is cycling.

The idea is that the fish will generate more ammonia and speed up the process.

However, fish should not be added to a tank until the procedure is complete.

A tank that is undergoing cycling is very unstable. Water conditions are constantly changing and the fish may not be able to tolerate these shifts. Cycling can cause the parameters to dip out of these ranges and create a very stressful environment for your fish.

While it may be tempting to add fish, it is best to wait for the natural processes to run their course. Your fish will certainly appreciate your patience.

Why is my aquarium cloudy?

Your aquarium water can become cloudy during the later stages of nitrification.

As the amount of nitrites increase the amount of oxidizing bacteria in your tank will also increase. These blooms will lead to murky and cloudy water. Once the nitrification runs its course the bloom should clear up on its own.

If the bloom does not clear there may be too much ammonia in the water.

What is new tank syndrome?

New tank syndrome is a term used for when fish are dying in your new tank. It is a very common problem for beginner fish keepers. A tank that is affected by new tank syndrome is likely poorly cycled and may still contain toxic amounts of ammonia in the water.

Fortunately new tank syndrome is easily prevented, you just need to completely cycle your tank before adding any fish.


When you buy your brand new fish tank you will be very eager to fill it with fish.

However your aquarium inhabitants will appreciate the time and energy you put it into this very important first step.

Just like in nature, cycling a fish tank can be very time consuming.

However it is well worth the wait for a mature and healthy fish tank.

When you cycle your tank you are witnessing a natural process at work right in your own home. This process occurs on a global scale in our watery world. With a little bit of patience and a lot of commitment you will be completing your first cycle before you know it.

Which method did you use to complete your most successful cycle? Let us know in the comments section below…