Goldfish tank water changes

Goldfish tank water changeWater changes are a key part of proper goldfish care.

Following the correct process of changing your goldfish’s tank water can be the difference between keeping your fish strong and healthy in water that is perfectly suited to its needs, and keeping your fish in poor quality water that is hazardous to its health…

In this post, we explain how often you should do a water change, how much water to change each time, and tell you some of the most common water change mistakes to avoid!

How often should you change your goldfish tank water?

Generally speaking, you should change some of your goldfish’s tank water once per week.

We say “generally speaking”, because you will need to do more regular water changes in some situations. For example, if your tank was not properly cycled before you added your fish, then you may need to change some tank water as often as daily, in order to keep ammonia levels under control. You should regularly use a test kit to keep an eye on your goldfish tank’s water parameters and change the water more often if you see spikes in ammonia or nitrite (see our post on cycling your tank).

How much water should you change each time?

You may have noticed that we said to change “some of” your goldfish’s tank water each week. You should not change all of the water at once! In a properly cycled tank, we suggest a weekly water change of around a third of your goldfish tank’s water.

How to do a water change

If you’re wondering how to actually go about changing your goldfish’s tank water then simply follow these 10 easy steps:

  1. Buy an aquarium vacuum (also known as a siphon or hose).
  2. Get a bucket.
  3. Turn off any equipment, such as your filter or air pump.
  4. Stick one end of the vacuum into the bucket and the other into the gravel at the bottom of your tank. Switch on the vaccum (or manually pump if necessary) to remove the water from the tank and transfer it to the bucket. This removes both water and waste from your tank at the same time.
  5. Keep vacuuming/pumping, while moving the hose around your tank to different areas of gravel, until around a third of your tank water has been removed (you may have to empty the bucket once or twice).
  6. Remove your filter and place it into the bucket of tank water – note: tank water, NOT tap water – gently squeeze out your filter sponges to remove any gunk that may be blocking your filter.
  7. Put your filter sponges back into your filter and re-install the filter in the tank.
  8. Wash any other equipment or decorations in the same way – in tank water, not tap water.
  9. Fill your bucket with tap water and – before adding it to your tank – treat the tap water with a product such as Seachem Prime. Follow the instructions on the bottle and do this for every bucket of tap water that you use.
  10. Gently and slowly pour the treated tap water into your tank. You should do this one bucket at a time and very slowly. Otherwise, the change in water condition and temperature may shock your fish.

The biggest water change mistakes goldfish keepers make

There are three major mistakes that new goldfish keepers often make when doing a water change:

  1. Washing their tank equipment and decorations in tap water – your filter sponges, gravel and decorations are where the good bacteria in your tank live! You should NEVER wash your filter, filter sponges, gravel or decorations in tap water, as it will kill this good bacteria! Instead, every time you do a water change, remove a bucket of tank water and use that to clean your tank’s equipment and decorations.
  2. Not treating tap water before adding it to the tank – water contains chemicals that can harm the colony of good bacteria in your tank. You should therefore use a product such as Seachem Prime whenever you do a water change. Prime treats your tap water and removes chemicals that could harm your fish or bacteria colony, such as chlorine, so that the water is safe to be added to your tank.
  3. Adding water too quickly – adding tap water too quickly or when it is at a very different temperature to your tank water can shock your fish. This can make your goldfish stressed, which lowers its immune system and increases the chance of it getting ill.

Is this how you go about your water changes? Do you have any comments, questions or tips that may help other goldfish keepers? Leave a comment below!

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  • Robert Sciolino

    I add the dechlorinator to the tank itself then i use a hose to add water to the tank. I have 2 75 gallon tanks. No buckets to fill for me

  • John Parkzit

    They have those nifty hoses that hook right up to your faucet too that **as it removes water from your fish tank it is also adding new water from your faucet. I forget the name.. it is on the edge of my tongue but I can’t think of it. Most times I will just use my gravel siphon for my butterfly fish tank and it works pretty good. That way, it is removing the dirtiest water and debris too while also doing a tank change. We have to remember that there is only so much oxygen in the water so we have to add new water.

  • M Selvaraaj Prabu

    Good article. Thanks for it Mr.GoldFishTank. I bought a 10 gallon tank for my son about 3 years back and had 3 (2 fantails and 1 blackmoor) in it. Everything was fine until last month. My son left to Canada for studies in Aug-13 and after a month I thought of cleaning the tank – and – did a full tank clean. Dame to me! Soon 1 fan tail got the swim bladder disease and died last week. Now the 2nd one is also just struggling to swim – lays on the bottom most of the time and lays on sides. I am worried and started googling. Today we bought a General Aid liquid for goldfish and added in the water. Waiting for my fish to get well. Any advice is appreciated.

  • desiree

    I forgot that I was out of the solution u put in the tank to make the tap water safe for my fish. If i let the water sit for 24 hours would it be ok to put them in it.

    • Shawn Murphy

      Not usually, although letting water sit for 24 hours does remove chlorine, it does not however remove chloramin, which is a form of ammonia used to treat drinking water for humans but is still toxic to fish. It would be wise to be sure to never run out of water conditioner.


      Ive done it before and right now my goldfish is fine

  • Laura Burnett Staynings

    So you leave them in the tank while you clean it out? I’m afraid I might vacuum them up.

    • TheGoldfishTank

      Hi Laura,

      There shouldn’t be any problem leaving them in the tank. Most aquarium vacuums are too small to suck up even a small goldfish.

      The best thing to do is to stick the end of the vacuum into the gravel (assuming you have gravel – just very near to the base if not). This keeps the vacuum well away from your fish and also means that you’re cleaning the area where most waste is collected.

      Hope that helps!



        What about the gravel? will it get sucked up?


    Ok so ive been keeping my goldfish in a bowl washing everything when I wash my tank and I just stick the goldfish in there right after its been treated. should I be concerned abut my fish. I mean he seems pretty healthy and hes 5 years old right now….