How To Cycle An Aquarium (Step By Step Beginners Guide)

The most important thing to know when setting up a new fish tank is how to cycle the aquarium. Every new aquarium needs to go through the Nitrogen Cycle to provide a thriving ecosystem for your aquatic pets and plants. Here’s everything you need to know about cycling a fish tank.

What Is The Nitrogen Cycle?

The nitrogen cycle is a process whereby various bacteria convert harmful waste. There are four main steps:

  1. The decay of waste products created by fish, plants, and invertebrates, as well as dead organisms or uneaten food.
  2. Nitrosomonas (a bacteria) converts ammonia to nitrite.
  3. Another set of bacteria called Nitrobacter converts the nitrite into nitrate, which is less toxic than nitrite.
  4. Anaerobic conditions then convert nitrate into nitrogen gas. Though this last step is not common in most aquariums, frequent water changes help dilute nitrate.

Why Do Fish Tanks Need To Go Through The Nitrogen Cycle?

Fish waste releases ammonia in the water which in turn can kill fish and other aquatic animals. The Nitrogen Cycle helps prevent ammonia build-up. It’s a process whereby beneficial bacteria in your tank and filter media, converts ammonia (toxic) to nitrite (toxic), then nitrite to nitrates (non-toxic).

Now although nitrates are non-toxic, a buildup can cause loss of appetite and discoloration in fish. So, water changes are necessary to reduce the amount of nitrate in the aquarium. The Nitrogen Cycle is an important part of establishing a healthy environment for fish, snails, shrimp, etc.

How Long Does The Nitrogen Cycle Take?

In general, the nitrogen cycle takes between six to seven weeks to complete. You need to monitor the cycle every other day using an aquarium test kit to check the ammonia, nitrites, nitrates, and pH levels.

Using the testing kit, you can tell the levels of ammonia, nitrite, and nitrate. Once the testing apparatus no longer detects nitrites, it is safe to assume that you can add your fish.

The length of time it takes to cycle an aquarium depends on the following:

  • The amount of ammonia produced during the cycling period.
  • The efficiency of the biological filtration.
  • Whether you use a live rock or live plants during the process.
  • Whether you boost bacteria colonies with additives and bio media.

How To Do A Fishless Cycle


A fishless cycle is a process of growing a colony of ‘good bacteria’ in an aquarium filter before introducing any fish to the tank. By adding a source of ammonia, you can encourage the growth of the bacteria that process ammonia to nitrite, and nitrite to nitrate.

To start, drop a few bits of fish food into the tank. Add a bit of food every 12 hours as if you were feeding fish. Once the fish flakes start to decompose, it releases ammonia into the tank.

Using the test kit, check the level of ammonia in the tank every 2-3 days. The reading should be about 3ppm. If there is insufficient ammonia add more fish food. Then, start to test every other day, and try to maintain the ammonia levels at 3ppm. At that point, Nitrosomonas begin to grow and consume the ammonia. Should the level of ammonia dip below 3ppm, add more fish flakes to keep the level stable. Do this for one week.

After one week, start to test for nitrites using the testing kit. Once you detect nitrites, you can assume that the the nitrogen cycle is on the way.

Continue to test for ammonia and nitrite over the next three weeks. Once the nitrite levels begin to decrease, start testing for nitrate. Once you detect nitrates, you’ll know that the cycle is near completion.

The nitrogen cycle is complete once you get a reading of zero for both nitrites and ammonia. Before you add your fish, ensure that the nitrate level is below 20ppm. If not, do a water change to reduce the amount of nitrate in the fish tank.

Use a siphon, substrate vacuum, or hose to remove trapped food. Once everything is in order you can start to add fish little by little. Adding 2-3 fish per week is safe.

How To Speed Up Nitrogen Cycle

If you don’t have access to an already established (nitrogen cycled aquarium), then you can use products like API Quick Start Nitrifying Bacteria to speed up the nitrogen cycle in your fish tank.

If you already have a cycled tank with some sort of filter, you can remove the foam block or floss insert and place it in the new tank. These components already have nitrifying bacteria.

If you have an established tank with an under gravel filter, nitrifying bacteria will be present in the gravel. Transfer some gravel from the cycled tank to the new tank if it also has an under gravel filter.

If you have a box or sponge filter, connect it to an established aquarium and leave it running for a week. Bacteria will colonize the new filter, and you can then transfer it to your new tank.

Never transfer anything from a tank that’s known to be contaminated with harmful organisms or where there appears to be sick or dying fish.

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Last update on 2020-10-27 / Affiliate links / As an Amazon Associate this website may earn from qualifying purchases.