Goldfish Tank Setup: Cycling your goldfish tank

So you’ve decided to get a pet goldfish, you’ve bought a suitable tank, a large enough filter, and maybe some food, plants and decorations… all you have to do now is rush down to the pet shop and pick out your new fish, right? Wrong!

There’s one more very important step to goldfish tank setup and proper goldfish care, which you must follow in order to give your fish the best chance of living a healthy, happy and long life.

That step is called “cycling” your tank. Here’s what cycling is, and what you need to do…

What does “cycling a goldfish tank” mean?!

This may sound complicated at first, but don’t worry, it’s not too bad really and cycling your goldfish tank is very important – so stick with it!

Like all animals, goldfish produce waste. This isn’t a problem for them in the wild, as there is so much water in rivers and in the sea that their waste gets diluted and doesn’t do them any harm. However, in a tank – even quite a big tank – their waste will build up in the water (even if you do lots of water changes) and will damage their health.

This is where “cycling” comes in! When we talk about “cycling” a goldfish tank, what we’re referring to is building up a colony of “good bacteria” in your tank – mainly in your filter and gravel. These bacteria take your water through something called “the Nitrogen Cycle” and make it safe for your fish to live in.

There are two stages to this process:

1. Ammonia (a harmful chemical in fish waste) is turned into something called “nitrite”

This is good – because ammonia is very bad for your fish, so you want to get rid of it! Unfortunately, nitrite isn’t very good for them either. So we need to get rid of that too!

2. Nitrite gets turned into something called “nitrate”

Bacteria to the rescue again! Fortunately, bacteria can turn nitrite into something called “nitrate“. Now, nitrate is still not “good” for your fish, but it’s a lot less bad than ammonia and nitrite, and is only harmful in high quantities. (You’ll prevent nitrate getting too high by doing regular water changes.)

How to cycle a goldfish tank

Cycling a goldfish tank is really easy. All you need to do is set up your tank as if the fish were already in it – including adding gravel (if you want gravel!), installing a suitable aquarium filter and switching your filter on – then regularly add ammonia until your water tests show zero ammonia, zero nitrite and some nitrate.

Great! Wait, what? tests?! Yeah, this will feel a bit like chemistry class, but it’s important (and can be fun!). You need to use a test kit to test your water during the cycling process to see whether you’ve built up a bacteria colony that are getting rid of your ammonia and nitrite and turning them into nitrate. Don’t worry – they come with instructions and are really easy to use!

As well as a test kit, you’ll also need a source of ammonia.

Once you have your ammonia and your test kit – and you’ve setup your tank, with water added and the filter running – simply start adding one drop of ammonia per day for every gallon of water in your tank.

After a few days, test your water for ammonia and for nitrites. You should see quite a high ammonia reading (as you’re adding it!) and maybe some nitrites. if you don’t see any nitrites then don’t worry – just keep adding ammonia each day until you do.

Once you get a nitrite reading, all you have to do is keep adding ammonia and start testing for nitrates. After a few more days – or possibly weeks – you’ll see nitrates start to appear. Once this happens, keep adding ammonia and keep testing for all three substances. Eventually, you will see ammonia go down to zero and nitrites go down to zero, while nitrate readings stay high. Congratulations! Your tank is cycled!

What to do once your tank is cycled

  • Keep adding ammonia until the day before you get your fish – this is to keep the bacteria “fed” until your fish arrive and start adding their own ammonia (waste)
  • Do a water change of at least 50% to lower nitrates before you add any fish
  • Add your fish one at a time and continue testing your water – adding too many fish could upset the delicate balance of bacteria in your tank and see ammonia or nitrites reappear

And that’s it! Follow this simple (even if it may sound a bit complicated at first!) cycling process and you’ll soon be ready to buy your first goldfish!

We know cycling can be frustrating, and may mean that you can’t bring home that first fish as soon as you’d like though, so see below for a few tips on how to speed the process up, a few warnings to make sure you don’t mess the process up, and a summary of the main points presented in this article.

Speeding up the cycling process

There are a few things that may help speed up the cycling of a fish tank.

  • High temperature: Keeping your water slightly warmer than you would usually can make bacteria reproduce more quickly.
  • Bacteria in a bottle: You can buy bacteria in a bottle to help start your bacteria colony. How well these products work is debatable. And don’t believe what they say about adding fish “instantly”. you should still add ammonia and test your water until it is properly cycled!
  • Borrow bacteria from a friend! “Eww…what?!”… Don’t worry, it’s not as disgusting as it sounds! We just mean that, if you know someone who already has a well established, cycled fish tank (which is free of diseases) then it can help to “seed” your tank with bacteria from there tank. Simply take some of their gravel or cut a piece of their filter sponge off and add it to your own tank to transfer the bacteria!


  • DO NOT clean your filter or gravel in tap water at any point during the cycling process or afterwards. Tap water contains chemicals that are harmful to the good bacteria in your tank. Your filter sponges and your gravel are where the bacteria live and you don’t want to kill them! You should only ever clean your filter in water from your tank.
  • DO NOT listen to people who tell you to add “feeder” fish straight away as a source of ammonia. It’s cruel and they will almost certainly die. The above “fishless” cycle using bottled ammonia is much kinder!
  • DO NOT add too many fish at once – as mentioned above, this can ruin all of your hard work.
  • STAY PATIENT! Cycling a fish tank can take weeks – or even longer – but it’s much better than having unhealthy, unhappy fish that die soon after you buy them.


We know that cycling is a complicated topic and can seem confusing at first, but it’s important to realize that goldfish care starts before you get your fish, not when you get your fish. Here are the main things to remember:

  • Cycling is a vital part of keeping healthy fish
  • Cycling should be done before adding any fish
  • Cycling means adding ammonia until enough bacteria exists to “eat” all of the ammonia and nitrites in your tank
  • You start the process off by adding bottled ammonia to your tank
  • Your tank should be fully setup – including the filter running – when you do this
  • You measure the levels of ammonia, nitrites and nitrates using a test kit.
  • You can tell when enough bacteria exists by test readings that show zero ammonia, zero nitrite and some nitrate
  • Once your tank is cycled, it’s important to add fish slowly (not too many at once) and still do regular water changes

2 thoughts on “Goldfish Tank Setup: Cycling your goldfish tank”

  1. Hi, I got a fish at the carnival today and then went to Walmart and bought all the stuff I thought I needed; a big tank with a filter and all the chemicals and such, except I had no idea about cycling until getting on this website and I already have the fish! I had to get it out of the little bag it came in so I have my Lil guy all set up. I am however worried about ammonia. Can I cycle the tank with him in it? How do I keep him safe? Thanks for your help.

    • Hi Donovan, thanks for your comment,
      It is great to hear that you have gone out of your way to provide your goldfish with the best care.
      Cycling can be done while the fish is in the tank yes.
      Don’t add ammonia or anything, just look after your fish as you normally would, but feed them less, maybe only 3 times a week to start.
      and keep a close eye on water parameters, probably do a 20% water change every 4 days to keep the water chemistry clean.
      At any time if you test and find ammonia or nitrites, do a water change.

      You can actually purchase beneficial bacteria from pet stores which helps to cycle your tank too, just add some of this to your tank over the next few days.
      hope this helps, and good luck 🙂


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