How do I cycle my new aquarium?

Level: Beginner

This is a beginners guide to cycling a new aquarium.

Who should use these instructions?

Everyone who keeps fish.

You should use these instructions when:

  • You are setting up a brand new aquarium
  • If you are setting up a previously used aquarium that hasn’t recently had water in it (for more than a couple hours)
  • Or, if you have recently emptied and sanitized your aquarium
    • Only sanitize an aquarium to eliminate disease, mold or harmful substances
      • I will provide a guide on this in the future.

Why is it called Cycling an Aquarium?

An aquarium is an emulation of a natural environment. In nature, the cycles of creatures being eaten, processed through other creatures; aquatic plants absorbing nutrients and processing light into energy through photosynthesis; and water evaporating and falling as rain are all part of a natural series of “cycles”. When we talk about cycling an aquarium, we are referring specifically to the cycle through which beneficial micro-bacteria convert ammonia (NH3) and nitrites (NO2) into nitrates (NO3).

Why should I cycle my aquarium?

Ammonia, nitrites and nitrates are all harmful to fish and other aquatic organisms. So, the goal of cycling an aquarium (and doing water changes) is to produce and maintain a population of beneficial bacteria that converts ammonia into the less harmful nitrites and nitrates, which can then be removed through water removal and replacement (AKA water changes).

Cycling a fish tank can keep your fish from rapidly dying. If you just fill an aquarium with water and put fish in it, they will likely die within 24 hours. This is because of the volume of ammonia in the water increasing to lethal levels (also called an ammonia spike). To prevent this from happening, fish need beneficial micro-bacteria in the water that converts toxins like ammonia and nitrites into nitrates that are easily removed when Doing Water Changes.

How is aquarium cycling done?

The process of cycling an aquarium involves Live Bacteria populating and reproducing to sufficient numbers to adequately kickstart the ongoing biological cycle of chemical conversion (mentioned above).

Cycling can technically be done with or without fish in the tank. But, we do not advise you do a fish-in-tank cycle. Starting a new aquarium cycle, in a tank that contains fish, is risky and poses a high likelihood of your fish becoming sick and dying .

Instead, the most recommended method is a fishless cycle.

There are two common ways to do a fishless cycle:


  • By purchasing/receiving various products or media (filter pads/sponges/substrate etc.) that contain live bacteria and adding to your tank.
    • Follow instructions provided to you and use a test kit (more details below) to verify when your tank has been fully cycled.
      • This can be instant or may up to a couple weeks


  • By adding a small pinch of fish food to your new tank
    • As the fish food breaks down, it produces a very small amount of ammonia and nitrite
      • This attracts bacteria to begin populating and reproducing in your aquarium, due to the presence of a food source

This process can take several weeks or more.

How do I know when my aquarium cycle is complete?

Your aquarium cycling doesn’t really end, you just get to stop heavily monitoring it when your aquarium ecosystem is balanced.

Use an aquarium water test kit to know when micro-bacteria is converting the full bio-load (ammonia and nitrites) into nitrates.
When the test kit shows nitrates and no ammonia or nitrites, your aquarium is ready to add fish.

Begin Doing Water Changes to remove nitrate build-up from harming your fish.


Want more information? 

Read the Beginner’s Guide to Fish and Aquarium Care

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