How to Start Nitrogen Cycle in an Aquarium

Unlike in their natural habitats, tropical fish in aquariums are more susceptible to excessive ammonia and nitrification in the water. 

These result from the closed environment in a fish tank coupled with factors such as overcrowding and overfeeding. 

Fish generate waste which they release into the water. This waste produces ammonia, which, if not put in check, will kill your fish.

This is where the nitrogen cycle comes in. 

In an aquarium, this cycle refers to a biochemical process that ensures a continuous breakdown of nitrogenous compounds from ammonium to nitrite, then nitrate. 

It relies on establishing beneficial bacteria in the tank and filter media to help in the breakdown process. 

Here’s an in-depth look at how to start the nitrogen cycle in your aquarium and provide a healthy environment for your aquatic pets.

But first, a brief look at what happens when cycling your aquarium:

The Nitrogen Cycle Timeline

The nitrogen cycle occurs in three distinct stages:

Stage #1: Ammonia

Typically ammonia is introduced into your aquarium through fish waste and decomposing food.

It then builds up until the nitrifying bacteria that feed on it start to grow. This is usually signified by your tank going cloudy.

Stage #2: Nitrites

Nitrite concentrations start to rise as ammonia declines. 

This results from the beneficial bacteria — called Nitrosomonas bacteria feeding on ammonia and turning it into nitrite.

This usually happens towards the end of the first week or the second week of the cycle process.

Stage #3: Nitrates

Nitrates are the product of the cycling process. They are a product of Nitrobacter bacteria feeding on nitrites and converting them into nitrates.

When this happens, the nitrogen cycle is complete. 

Note, however, that though nitrates are not toxic to fish in low concentrations, they can quickly build up and become harmful. 

To maintain acceptable levels, you will need to perform partial water changes (every one to four weeks) depending on the number of fish in the tank and tank size.

If you have a fresh water tank, you can also introduce some aquarium plants to help use up some of the nitrates.

Nitrogen Cycle
Related: Nitrate Poisoning in Aquarium Fish – Possible Causes and Remedies

How to Cycle Your Aquarium

There are two main methods to start the nitrogen cycle:

  1. Fishless Cycling – where the cycle is established without using any fish. 
  2. Nitrogen Cycle by Fish – where live fish are used to introduce ammonium into the aquarium, kickstarting the nitrification process. 

However, as you shall see, one of these methods takes a heavy toll on your aquatic pets. 

The Fishless Cycle

This is the best-recommended way to start a nitrogen cycle in your new tank. It doesn’t harm your aquatic pets and is by far more humane.

Step #1: Introduce Ammonia to Your Tank

The first thing you need to do is to establish an environment where ammonia is produced. To do this, drop some fish food into the tank. 

As the food decays, it will start releasing ammonium into your aquarium.

Step #2: Test Your Ammonia Levels

To do this effectively, you will need to grab yourself a test kit and monitor the ammonium levels every day. Aim for at least 3ppm (parts per million).

If there’s not enough, add some more food flakes and allow them to decay.

Nitrosomonas bacteria will start to grow and consume the ammonia at around 3ppm, causing it to drop in concentration. But you need to maintain the concentration for them to thrive. So add some more food every 12 hours and continue testing your water every other day.

Do this for at least a week.

Test Your Ammonia Levels

Step #3: Test for Nitrites

After the first two weeks of testing for nitrite concentration, If you can’t detect nitrites in the aquarium water, you’ll know that your biological filtration is established. 

The bacteria are successfully converting the ammonia to nitrite. 

Step #4: Test for Nitrates

After about 2 weeks of more testing, you will notice the nitrite levels start to drop.

This means that your biological filtration is almost complete, and it’s time to begin testing for Nitrate.

Once the ammonia and nitrite levels hit zero, the cycle is successful.

Step #5: Introduce Your Fish

When you can no longer detect ammonia and nitrites, and the nitrates are at a safe level, then it’s safe for your pets to move in!

Introduce Your Fish

Add the fish after you’ve confirmed that the nitrate level is not too high. If the reading is above 40ppm, do some water changes to bring the levels back down.

You’ll also need to take things slow — a few fishes at a time. Then wait at least a week before adding some more.

It’s best if you also cleaned any substrate before adding fish. Some decaying food might still be trapped there. When disturbed, it could expose your fish to ammonia.

Related: How to Change Aquarium Substrate (The Right Way)

Starting the Nitrogen Cycle with Fish

We preface this by stating that this is the less preferred method to cycle your aquariums.

It involves exposing your fish to ammonia and nitrites, putting them at risk.

But if you made the mistake of buying a new aquarium and fish on the same day, this might be your only option.

Step #1: Introduce a Few Hardy Fish

The goal is to populate your aquarium with fish that will add ammonia to the water.

It’s best if they can also tolerate high levels of ammonia long enough for the beneficial bacteria to grow.

To improve the chances of your fish surviving, shoot for one or two for every 10 gallons of water.

Any more, and they might produce too much waste leading to an ammonia spike that might kill them.

The best-recommended breed of fish for cycling include:

Step #2: Feed Your Fish

Feeding should be done sparingly — once every two days. You don’t want to overfeed them and trigger an ammonia spike from the excess waste.

But you still need to ensure there’s enough ammonia and nitrite for the bacteria to feed on.

Also, provide only moderately sized meals. Remember, uneaten food will generate even more toxins.

Step #3: Change the Water

During the first cycle, your fish will be exposed to toxic levels of ammonia and nitrites. Change the water regularly to keep toxin levels manageable.

Ideally, you should aim for a 10% -25% water change every two to threedays. This will help maintain a balance for your fish to survive while encouraging bacteria growth.

ONLY use de-chlorinated water. Chlorinated tap water will kill the bacteria and ruin the entire process.

Related: How to Remineralize RO Water for Aquarium

Step #4: Check Your Toxin Levels

You will need to monitor the ammonia, nitrite, and nitrate concentration levels using a testing kit.

While you can get away with not testing for ammonia, it is advisable because it’s highly toxic — a spike could kill your fish.

Tracking your nitrite levels will confirm that the biological cycle is established and shows its progression. And when you can no longer detect any nitrites in the water, it’s a sign that the cycling process has ended.

Step #5: Add Some More Fish

When you can no longer detect any more nitrite in the tank, you can start adding fish to the aquarium.

However, you will need to do this slowly — only introducing one to twofish at a time with at least a week between additions.

Too many fish at once will cause a spike leading to stress and possibly diseases.

Also, don’t add fish unless you’ve confirmed that the nitrate levels are safe. 

Expand Your Fishkeeping Knowledge

The above are the two most popular ways to start a nitrogen cycle in your aquarium. 

The fishless cycle has the benefit of being more humane, but it takes a few weeks longer. And though using fish is faster, it might end up costing the lives of your aquatic pets. 

But these are not the only variables to consider. 

For more information on the nitrogen cycle, fish tanks, and the fishkeeping hobby, visit Aquarium Sphere—Your one-stop resource for all aquarium and fishkeeping advice. 

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