How to Cycle a Fish Tank

How to Cycle a Fish Tank
A Fish Tank being Cycled

Cycling your aquarium is a crucial step in creating a stable and healthy environment for your fish and other aquatic inhabitants. It helps establish beneficial bacteria through the nitrogen cycle, prevents fish stress and disease, prevents algae blooms, and contributes to the long-term success of your fish tank. In fact, if you start adding lots of fish to the tank without properly cycling your tank, it can cause an imbalance in the water parameters and eventually cause harm to the fish.

How to Cycle a Fish Tank

Here’s a step-by-step guide on how to cycle a fish tank:

1. Set Up the Tank

  • Place the aquarium in a suitable location away from direct sunlight and drafts.
  • Add a layer of substrate to the tank floor, such as gravel or sand.
  • Fill the tank with dechlorinated water to the recommended level.
  • Decorate the tank with ornaments and plants.
  • Install a filtration system. This is important because the water flow from the filtration will oxygenate the water, which will help get the beneficial bacteria to get established later on.

2. Add Ammonia

  • In a fishless cycle, add a controlled amount of pure ammonia, such as Fritz Ammonia, to the tank. This simulates the waste produced by fish. Aim for an ammonia concentration of around 2-4 ppm (parts per million). As an alternative to ammonia, you can add fish food as well. Even though it will take longer, the food will break down and produce ammonia eventually.
  • In order to cycle with fish, the fish will consume the food and produce waste, which will also turn into ammonia. If fish is used for the cycling process, only small fish with small bioload should be used. They should be hardy fish that can withstand minor fluctuations in water parameters as well. Zebra Danio, Guppies, Mollies, Platies, and Goldfish are often used for cycling a tank. Cycling a tank with fish requires careful consideration, as these hardy fish can also suffer from poor water parameters.

3. Add Beneficial Bacteria

  • Introduce a source of beneficial bacteria to the tank. This can be in the form of a bacterial supplement or by using a small amount of established filter media from a cycled tank.
  • The bacteria will help convert harmful ammonia into less toxic substances.
  • This is an optional step, as beneficial bacteria will establish itself naturally. However, this is a recommended step if you wish to speed up the cycling process.

4. The Ammonia Eating Bacteria Appears

  • Allow time for the ammonia-eating bacteria (Nitrosomonas) to establish in the tank. This process may take a few days to a couple of weeks.
  • Monitor the ammonia levels and ensure they are decreasing.

5. The Nitrite Eating Bacteria Appears

  • As the ammonia levels start to decrease, nitrites will begin to accumulate. This is a natural part of the nitrogen cycle.
  • Allow the nitrite-eating bacteria (Nitrobacter) to establish, which will convert nitrites into nitrates.

6. Test Water Parameters

  • Regularly test the water parameters using a test kit. Monitor ammonia, nitrite, nitrate, pH, and temperature.
  • The goal is to see a spike in ammonia, followed by a spike in nitrite, and finally, a decrease in both as nitrates increase.

7. Introduce Fish Gradually

  • Once ammonia and nitrite levels have dropped to near zero, and nitrates are present, the tank is cycled and ready for fish.
  • Introduce a small number of fish gradually to avoid overloading the system.
  • Continue to monitor water parameters and perform regular partial water changes to maintain a healthy environment.


Here are some frequently asked questions related to cycling a fish tank:

What is the Aquarium Nitrogen Cycle?

The aquarium nitrogen cycle is a crucial biological process that occurs in fish tanks and other aquatic environments. It involves the conversion of toxic ammonia (NH3) produced by fish waste and decaying organic matter into less harmful substances through the action of beneficial bacteria. The nitrogen cycle consists of several key stages:

Ammonia Production:

Fish produce waste, and uneaten food and decaying plant matter also contribute to the release of ammonia (NH3) into the water. Ammonia is highly toxic to fish and can be deadly in high concentrations.

Ammonia-oxidizing Bacteria (Nitrosomonas):

The first group of beneficial bacteria, Nitrosomonas, convert ammonia into nitrites (NO2-). This process is called ammonia oxidation.

NH3 + O2 → NO2- + H2O + 2H+

Nitrite Production:

Nitrites are also harmful to fish, albeit less toxic than ammonia. Nitrites can still cause health issues and stress for fish.

Nitrite-oxidizing Bacteria (Nitrobacter):

The second group of beneficial bacteria, Nitrobacter, further convert nitrites into nitrates (NO3-). This process is called nitrite oxidation.

NO2- + O2 → NO3- + H2O + 2H+

Nitrate Production:

Nitrates are less toxic than both ammonia and nitrites, but high concentrations can still be harmful to fish, especially in freshwater environments.

Denitrifying Bacteria:

Under anaerobic (low oxygen) conditions in certain parts of the substrate or in filter media, denitrifying bacteria can convert nitrates into nitrogen gas (N2) or other nitrogen compounds, completing the cycle.

2NO3- → N2 + 2O2

Water Changes:

Regular water changes help control nitrate levels by removing them from the system.

By establishing and maintaining a healthy population of these beneficial bacteria in your aquarium, you ensure that ammonia and nitrite levels are kept low, providing a safe and stable environment for your fish. This process is fundamental for the success of any aquarium, whether it’s a freshwater or marine system.

What Supplies Do I Need to Cycle an Aquarium?

To cycle an aquarium effectively, you’ll need a few essential supplies to create a suitable environment for beneficial bacteria to establish and thrive. While some items are optional, here’s a list of supplies you’ll need:

Aquarium Tank: Choose an appropriately sized aquarium for the type and number of fish you plan to keep.

Water Conditioner: Use a water conditioner to remove chlorine, chloramine, and other harmful substances from tap water before adding it to the aquarium.

Filter: A good-quality aquarium filter is essential for mechanical and biological filtration. It provides aeration for the entire tank and a surface for beneficial bacteria to colonize.

Test Kits: Invest in reliable water test kits to monitor ammonia, nitrite, nitrate, pH, and other water parameters. Regular testing is crucial during the cycling process.

Ammonia: If you’re using the fishless cycling method, you can add pure ammonia without additives to simulate the ammonia source for the bacteria. Fish food can also be a source of ammonia as well.

Beneficial Bacterial Starter: A beneficial bacterial starter will help kick start the nitrogen cycle. Adding filter material or other seeded material from an established tank can be effective. There are also beneficial bacterial starter products such as Fritz Turbo Start. While beneficial bacteria can establish itself naturally, a starter will help speed up this process.

How Long Does an Aquarium Take to Cycle?

The time it takes to cycle an aquarium can vary, and several factors influence the duration of the cycling process. On average, it typically takes anywhere from 4 to 8 weeks for a fish tank to fully cycle. However, some tanks might cycle faster or take longer depending on the specific conditions. Here are the factors that affect the cycling time:

Ammonia Source: The type and amount of ammonia introduced into the tank play a role. If you’re using a fishless cycling method with a controlled ammonia source, the process might be more predictable compared to cycling with fish.

Beneficial Bacteria Population: The establishment of beneficial bacteria colonies is a crucial aspect of the nitrogen cycle. The time it takes for these bacteria to multiply and form a stable population depends on factors like temperature, pH, and the surface area available for colonization (provided by filter media, substrate, and decorations).

Initial Bacterial Inoculation: Some aquarists use commercially available bacterial starter products, such as Fritz Turbo Start, to jumpstart the cycling process. These products contain live beneficial bacteria that can help speed up the establishment of the nitrogen cycle.

Water Parameters: The cycling process is influenced by water temperature, pH, and hardness. Warmer temperatures generally accelerate bacterial activity, but extreme conditions can also stress bacteria.The optimum temperature for freshwater nitrifiers is 86°F.

Fish Load: The number of fish in the tank and the type of fish can affect the cycling time. Overloading the tank with fish can result in higher ammonia production, potentially slowing down the establishment of the nitrogen cycle.

It’s crucial to be patient during the cycling process. Rushing the cycle can lead to imbalances and stress for the fish. Regularly test the water parameters and wait until both ammonia and nitrite levels consistently read zero before considering the aquarium fully cycled. After the cycle is complete, you can begin gradually adding fish to the tank.

Should I Do Water Changes During the Cycling Process?

Performing water changes during the cycling process is beneficial and can help create a healthier environment for the fish and the beneficial bacteria involved in the nitrogen cycle. While beneficial bacteria need time to establish and colonize the aquarium, water changes serve several important purposes such as ammonia and nitrite control, nitrate management, and removal of debris.

Here are some guidelines for water changes during the cycling process:

Frequency: Perform partial water changes as needed, especially if ammonia and nitrite levels become elevated. In the early stages of cycling, when ammonia and nitrite spikes are common, more frequent water changes may be necessary.

Amount: Aim for partial water changes of about 20% to 30% of the total aquarium volume. This helps maintain stability while reducing the concentration of harmful substances.

Water Conditioner: Always use a water conditioner to treat tap water before adding it to the aquarium. This removes chlorine, chloramine, and other potentially harmful chemicals. If a significant amount of untreated water is added to the tank, it can destroy the beneficial bacterial that is trying to get established.

Avoid Complete Water Changes: Avoid doing complete water changes, as this can disrupt the cycling process by removing beneficial bacteria. It’s best to change only a portion of the water at a time.

Monitoring water parameters regularly using a reliable test kit is crucial during the cycling process. This allows you to respond to any spikes in ammonia or nitrite levels promptly. Remember that patience is key, and the cycling process may take several weeks to complete.

Leave a Comment

Your email address will not be published. Required fields are marked *