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A Beginner’s Guide to Cycling Your Fish Aquarium

One secret for keeping aquarium fish? If you look after the water conditions, your fish will look after themselves. That’s where the nitrogen cycle comes in.

The tank cycling process involves building a beneficial bacteria ecosystem, which breaks down toxic chemicals like ammonia and nitrite, leaving only safe nitrate in the water.

The fish nitrogen cycle can seem daunting for beginners, but running a fishless cycle is relatively simple. For a step-by-step guide, read this article twice to ensure a thorough understanding of the nitrogen cycle and how to properly cycle your aquarium.

Tank Setup for Nitrogen Cycle

What’s Nitrogen Cycle and Why Does it Matter?

The Nitrogen Cycle is the process by which ammonia (NH3) is converted into nitrite and nitrite (NO2−) is converted into nitrate (NO3-) by bacteria called Nitrosomonas and Nitrobacter (1). It’s also known as nitrification, the biological cycle, or the break-in cycle.

This nitrogen cycle occurs constantly in the background of your fish tank. Fish produce ammonia through their waste, which is then converted by Nitrosomonas bacteria into nitrite and by Nitrobacter bacteria into nitrate. These bacteria require a consistent supply of oxygen and alkalinity to support the conversion process.

It’s crucial to maintain the nitrogen cycle as ammonia and nitrite are toxic to fish. Even low levels of these chemicals (above 0.5 ppm) can harm or kill your fish when reaches 2.0 ppm (2). Nitrate, the final byproduct, is harmless in lower concentrations (20-40 ppm). Therefore, the Nitrogen Cycle plays a critical role in maintaining a healthy aquarium environment for your fish.

The Fish Nitrogen Cycle Process

The nitrogen cycle, crucial to maintaining a healthy aquarium, involves the conversion of harmful ammonia to harmless nitrates by beneficial bacteria in three stages.

Step 1: Ammonia (Toxic)

The first stage in the nitrogen cycle is the creation of ammonia. The main source of ammonia in a fish tank is fish waste, rotting food, and dead plants. As ammonia levels rise, ammonia-eating bacteria called Nitrosomonas bacteria to appear and begin to consume the ammonia. This causes ammonia levels to decline.

Step 2: Nitrites (Toxic)

During stage 2 of the fish nitrogen cycle, nitrites are produced as Nitrosomonas convert ammonia. Nitrites are toxic to fish, but the cycle continues with the introduction of Nitrobacter bacteria. These bacteria consume nitrites and decrease the nitrite levels in the tank, indicating their presence.

Stage 3: Nitrates (harmless in lower concentrations)

The Nitrobacter bacteria in your aquarium convert nitrites into nitrates, which is the final product of the nitrogen cycle. It’s important to note that nitrates are harmless to fish in concentrations of up to 20-40 ppm. However, if the nitrate levels become too high, you can reduce them by performing a partial water change.

The nitrogen cycle describes the process of converting harmful ammonia into harmless nitrate. When setting up a new aquarium, cycling the tank helps to establish a population of nitrifying bacteria that can keep ammonia and nitrite levels at zero, making it safe for fish to be added.

What Happens If You Don’t Cycle a Fish Tank?

If you don’t cycle a fish tank, chemicals like ammonia and nitrite will build up to toxic levels and kill your fish.

Your fish will produce ammonia as waste and nitrifying bacteria will eventually develop. However, this process takes time, and during this period, your fish will be living in water with dangerous levels of ammonia and nitrates. This can lead to chemical burns on your fish’s gills and internal organ damage.

Allowing the nitrogen cycle process to occur naturally is risky, but some hobbyists choose to do so. This is called fish-in cycling, but it’s not recommended for beginners. Even a small mistake can harm your fish, making them sick or permanently damaging their immune system.

Performing a fishless cycle creates a balanced ecosystem that’s safe for your fish to inhabit.

How Long Does It Take to Cycle a Fish Tank?

It typically takes between four to six weeks to cycle a fish tank (3). The goal of the cycling process is to establish a population of nitrifying bacteria that will convert ammonia and nitrite to acceptable levels of nitrate.

It’s important to note that some local fish stores may advise running a fish cycle for a week before adding fish, and some even claim that a tank can be cycled in 24 hours. However, it’s important to keep in mind that nitrifying bacteria grow slowly, and there is no way to naturally speed up this process. To ensure a healthy ecosystem for your fish, patience is key.

If you’ve already purchased your fish and are just realizing you need to cycle the tank, don’t worry. You can perform an emergency fish-in cycle, which involves adding your fish to the tank while the cycling process is still underway. This method allows the beneficial bacteria to grow in the presence of your fish, ensuring a healthy aquarium environment.

How to Cycle Your Aquarium: The Safest Way

There are two ways to cycle your fish tank: a fishless cycle or a fish-in cycle.

I recommend a fishless cycle for anyone new to the hobby or anyone who wants to avoid exposing their fish to toxic levels of ammonia. This method is less stressful for your fish and ensures a healthy aquarium environment.

What Do You Need for a Fishless Cycle?

To begin a fishless cycle, you will need a few essential items:

  • Aquarium Test Kit
  • Ammonia
  • Water Conditioner/Dechlorinator

Aquarium Test Kit

The easiest way to check aquarium water parameters is by using an all-in-one testing kit. With this kit, you can test for ammonia, nitrite, nitrate, and pH. From these levels, you can infer whether you have beneficial bacteria in your tank. I recommend the API Freshwater Master Testing Kit. It contains all the tests you need and detailed instructions on how to run them.

API Freshwater Master Test Kit
API Freshwater Master Test Kit

Using the API Freshwater Master Testing Kit is simple:

  1. Take a small sample of water in a test tube and add a few drops of the test solution.
  2. The test solution will change the color of the water, depending on the concentration of the compound being tested for.
  3. Compare the color of the water to the test chart to identify the level of ammonia, nitrite, or nitrate in the water.


Ammonia is a natural byproduct of fish waste in an aquarium. Without fish, you need to introduce ammonia yourself.

The best way to introduce ammonia artificially into the fish tank is with pure ammonia. Make sure it’s uncolored and unscented as you don’t want anything extra going in the tank.

  • Fritz Pro Aquatics Ammonium Chloride is great for larger tanks.
  • Fritz Aquatics Fishless Fuel Ammonia Solution makes dosing easy for smaller tanks.

Another way to add ammonia is by putting fish food in and letting that rot and break down. But it’s easier to keep ammonia levels within the acceptable range if you have pure ammonia to add.

Water Conditioner

When setting up your aquarium, it’s likely that you’ll use tap water to fill it. However, tap water contains chemicals like chlorine and chloramine that can be harmful to both your fish and the beneficial bacteria that help to maintain a healthy tank (4). To ensure the safety of your fish and the health of your tank, it’s important to use a water conditioner to remove these chemicals.

The Fishless Cycle 7-Step Method: Setting up Your Tank

To ensure a successful fishless cycle, follow these 7 steps closely and thoroughly.

Step 1: Set up Your Fish Tank

To begin setting up your fish tank, assemble your heater, air pump, and filter. Next, add the substrate and plants. Finally, fill the tank with conditioned water.

Remember, everything must be in place before adding the fish. The beneficial bacteria (BB) that you’re trying to grow, need a home and require a consistent warm temperature. The BB will primarily colonize on the filter media (sponge and ceramic ring) and some in the substrate.

Step 2: Checking the pH Level of Your Fish Tank

pH is a measure of how acidic or alkaline your water is. A pH of 7 is considered neutral, and anything below 7 is considered acidic.

It’s important to check the pH of your aquarium water because beneficial bacteria can’t live in water with a pH of 6 or below. This is crucial for the nitrogen cycle.

A pH of below 7 can slow down the fish cycle, and a pH range of 7-7.5 is found to have the highest activity of beneficial bacteria (5). Once your tank is set up, make sure to check the pH.

According to Environmental Protection Agency (EPA) guidelines, the pH of tap water in the United States is between 6.5 to 8.5.

Make sure that the water you add to your tank falls within the appropriate pH range. If you find that your water is too acidic or alkaline, you can use a pH stabilizer to adjust it.

Throughout the fishless cycling process, check the pH regularly. Beneficial bacteria can produce acids that lower it. If the pH drops below 7, perform a 20% water change.

Another important factor to consider is the amount of bicarbonate (measured in kH or carbonate hardness) in your tank. This helps keep the pH stable and encourages the growth of nitrifying bacteria (6). Nitrifying bacteria in your tank produce acid, which can raise the pH. A healthy amount of bicarbonate absorbs the acid and keeps the pH stable.

Effects of pH on Nitrosomonas and Nitrobacter Enrichment Cultures
Effects of pH on Nitrosomonas and Nitrobacter Enrichment Cultures

The relationship between pH and kH is a little complicated, but in general, a higher kH will result in a slightly higher, but significantly more stable pH. With freshwater tanks, 4-8 dKH (degrees of carbonate hardness) is optimal.

Once the tank is set up and the pH is good to go, it’s time to kickstart the fishless nitrogen cycle.

Step 3: Adding Ammonia

To kickstart your nitrogen cycle, you need to introduce ammonia into your tank. One way to do this is by adding fish food and letting it decompose, but using pure ammonia is a more efficient and precise method.

When using pure ammonia, it’s essential to read the instructions on the product to determine the correct amount to add. As a general rule, one teaspoon of pure ammonia per 100 gallons of water will result in an ammonia content of 4 ppm.

For smaller tanks under 40 gallons, aim for an ammonia content of 2 ppm. For tanks 40 gallons or larger, aim for 4 ppm. To ensure accurate measurement, consider using a set of measuring spoons.

After adding the ammonia, wait for an hour for it to disperse throughout the water. Then, test the water to check the ammonia levels and follow the instructions for an accurate reading.

If the ammonia levels aren’t at the desired level, adjust them by adding more ammonia or performing a partial water change. Keep in mind that an ammonia content of over 5 ppm can slow down the nitrogen cycle and delay the process.

Step 4: Checking for Ammonia-Eating Bacteria (Nitrosomonas)

Once the ammonia levels in your aquarium are dropping, it’s a sign that the ammonia-eating bacteria have appeared. To confirm, test the tank water for nitrites. Nitrosomonas eat ammonia and produce nitrites. If nitrites are present, then Nitrosomonas are present in your aquarium.

To continue the cycle, add 50% of the original ammonia amount added on day 1, as long as it doesn’t bring the total ammonia content to 5 ppm. This will keep Nitrosomonas busy converting the toxic ammonia.

Test your tank water daily for both ammonia and nitrites. Wait for nitrite levels to drop, which could take a few more weeks.

Step 5: Checking for Nitrite-Eating Bacteria (Nitrobacter)

To ensure the presence of nitrite-eating bacteria (Nitrobacter) in your aquarium, test for nitrates once nitrite levels begin to drop. Nitrobacter converts nitrites into nitrates as a by-product, so if nitrites are decreasing and nitrates are present, your cycle is in its final stage.

To maintain the cycle, add another half dose of ammonia. Fish will provide ammonia once they are introduced to the tank, but without them, it’s necessary to keep the cycle running artificially.

Continue testing daily, recording ammonia (one hour after adding ammonia), nitrite, and nitrate levels. Your cycle is complete when tests show zero or near zero (around 0.2 ppm is acceptable) ammonia and nitrites, 24 hours after adding ammonia.

Step 6: One Last Test

To ensure your fishless cycle is running smoothly, it’s important to conduct one final test before adding your fish. This test confirms that the aquarium is safe for your fish and that the beneficial bacteria are breaking down toxic ammonia and nitrites as quickly as they are produced.

To conduct the test, add a full dose of ammonia – the same amount you added on day one. After 24 hours, test your water for ammonia, nitrites, and nitrates. If the results show zero for ammonia and nitrites, it’s a sign that your denitrifying bacteria have done their job. Note the nitrate level as well.

However, if the results don’t read zero, it’s time for more patience. Keep testing the tank as the beneficial bacteria are still emerging. As long as there’s less than 0.2 ppm of ammonia and nitrite 24 hours after adding ammonia, your fishless cycle is running smoothly.

Step 7: Adding fish to your tank

The final step in setting up your aquarium is adding your fish. You have now created a healthy ecosystem for them to thrive in.

Before adding your fish, it’s crucial to acclimate them to the tank water. The water they come packaged in will have a different chemical makeup, and failing to acclimate them can cause stress and even death.

It’s important to keep in mind that while beneficial bacteria are consuming ammonia and nitrites, nitrates must be removed through water changes. Nitrate levels should be monitored and kept between 20 ppm to 40 ppm, depending on the type of fish. If nitrate levels are above 20 ppm, perform a partial water change before adding your fish. A 50% water change will reduce nitrates to 10 ppm.

Can I Speed Up the Cycling Time?

To speed up the fish cycling process, consider using filter media from an established tank or taking substrate from it to provide a readymade bacteria colony. You can also “season” a new filter by attaching it to an established tank.

Live plants in your tank can also encourage the growth of nitrifying bacteria. If possible, use plants from an established tank.

Water temperature is also an important factor. Nitrifying bacteria are most active in the range of 77°F – 85°F (7). However, keep in mind that temperatures below 70°F will slow down the cycle. Remember to keep the water within a range that is suitable for the fish you will be adding.

Wrapping Up…

Congratulations on successfully cycling your aquarium. You have now created a healthy and stable environment for your fish to thrive in.

Remember that the nitrogen cycle will continue to run in the background, with ammonia being added naturally from fish waste and consumed by the bacteria you encouraged to grow.

However, it’s important to keep a close eye on the levels of ammonia and nitrite. If these levels rise, a water change may be necessary to bring them down. In smaller tanks, chemical levels can spike rapidly, so be sure to test the water weekly and perform regular tank maintenance.

Enjoy the healthy environment you have created for your fish and their beneficial bacteria.

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