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How to Cycle a Fish Tank

how to cycle a fish tank

If you’re setting up your fish tank from scratch, learning how to cycle a fish tank is the most crucial element in preparing a safe and habitable environment for your new fish. In the natural world, fish don’t live in crystal clear tap water; they thrive in ecosystems made up of plants, other animals and bacteria. The same applies to your pet fish, and cycling their fish tank or aquarium is the way you can ensure their water has the qualities they need to survive. 

The good news is cycling a fish tank is simple once you know how to. Here at Pond Planet, we’ve created a guide to fish tank cycling, including how to cycle a fish tank and how to change fish tank water without killing your fish!

What is cycling a fish tank?

First thing’s first, what is cycling a fish tank exactly and why do we need to do it? 

Cycling refers to the nitrogen cycle, which, in regards to fish tanks and aquariums, means the process of establishing good bacteria in the water of your tank in order for your fish to survive. When you feed your fish, they produce ammonia in their waste which is toxic to them. They need good bacteria and plants to absorb this toxic waste in their tank and convert it first into nitrites (also toxic) and eventually to non-toxic nitrates.  

When you have a brand new tank, this good bacteria hasn’t had the chance to grow and therefore cannot absorb the toxic ammonia. A functional ecosystem in your tank takes a while to develop, which is where cycling comes in. 

How to cycle a fish tank

So, how do you cycle an aquarium or fish tank? There are a few different ways you can cycle your fish tank, so we’ve picked out two of the simplest and most effective that we recommend for both beginners or experienced tank owners.

In-Fish Cycling

In-fish cycling is one of the most popular methods of cycling a fish tank. It refers to the process of keeping only a couple of resilient fish species in the tank at the beginning and slowly increasing the number as you complete the nitrogen cycle.

The steps:

  • Set up your aquarium or tank with all your chosen plants, rocks and filtration system.
  • Add several of your most hardy fish from species that will be able to survive through the initial period of high toxin levels before the good bacteria can grow.
  • Feed your fish very lightly to begin with, aiming for once every other day. Ensure you don’t overfeed, as excess food will turn toxic along with the toxic waste produced by your fish.
  • Every few days, replace approximately one-quarter of your tank’s water to ensure toxins levels do not rise too high for your fish to survive. This is key when it comes to how to change fish tank water without killing the fish! If you have an existing tank, you can speed up this tank cycling process by adding media from this to your fresh tank to help spread good bacteria. 
  • Using an aquarium test kit, check your tank water every few days to measure the levels of ammonia or nitrite present. If they reach above 0.2ppm, do an immediate partial water change.
  • Once the ammonia and nitrite levels in your tank are undetectable or as close to as possible, you can start to gradually add the rest of your fish in stages, following this same process. The cycling process is complete when all your fish are able to feed normally and toxin levels stay at zero because the good bacteria is doing its job.

Plant Cycling

Alternatively, another method you can try is plant cycling. When it comes to how to cycle an aquarium, plant cycling is an effective and natural method that is ideal if you have your tank for a while before you get your first fish. Plants not only look great in your tank but can play a large role in creating a natural ecosystem. By including some live aquarium plants before your fish, you can encourage the growth of good bacteria before your fish enter. 

You’ll know when the nitrogen cycle is complete in your tank when your aquarium plants start to produce new growth, as this means they are converting toxins into new leaves. You can start adding your fish at this point and follow the same steps as you would in-fish cycling: feed them sparingly at first and test the water to check ammonia and nitrite levels are low enough.

How long to cycle a fish tank

When it comes to how long to cycle a fish tank, the time may vary depending on a variety of factors. Generally speaking, it could take anywhere between two to eight weeks. If you’re unsure, there are two clear identifiers you can look out for to know when your cycle is finished:

  1. The ammonia and nitrite levels in your tank have remained at zero for two weeks.
  2. Your tank or aquarium can go for two weeks with only once-a-week partial water changes.

Keep an eye out for these signs and you’ll know whether your cycle is complete! Remember, if you add more fish or make changes to your tank’s ecosystem, you’ll need to keep checking on your ammonia and nitrite levels to ensure your fish are able to survive and thrive.

Now you know how to cycle a fish tank, you’re ready to start! You can find all the aquarium supplies you need big and small in our collection here at Pond Planet, including aquarium starter kits to help you with the process.

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