Goldfish tank water changes

Water changes are a key part of proper goldfish care. But what is involved in changing goldfish water?

Following the correct process 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 mistakes people make when changing goldfish water.

Changing goldfish water

How often should you change your goldfish’s 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 that measures ammonia, as well as nitrite and nitrate, to monitor your water parameters. 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. Never keep your goldfish in untreated tap water.
  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.

18 thoughts on “Goldfish tank water changes”

  1. Big problem keeping ONE fancy goldfish. Sources say I need a minimum of 20-30 gallons. That’s a lot of water to change weekly. Add the amount of water treatment chemicals and medication – massive and expensive, inconvenient. Contacted API and found out that Growth Inhibiting Hormone (GIH) is not safe for the fish. Stunting will make fish sick. Eventually die. Scientific fact not myth. They must grow to max. size to be healthy. (8″). That’s a lot of work. No wonder children get disappointed when goldfish die. Very popular pet. Any advice to raise goldfish easier would be appreciated.

    • Hi Jack – thanks for your message. You’re right, keeping a goldfish can be a lot of work. Though this is true of most pets. All we would say is that goldfish-keepers often don’t think of it as “work”. We enjoy looking after our fish, which includes looking after their tank and water. If you don’t enjoy the cleaning, testing, feeding, etc, then maybe fish-keeping isn’t for you? Though we hope that you come to see the fun in it!

      Just one final point – you shouldn’t have to change ALL of your tank water every week. Or anywhere close, assuming your tank is properly cycled.

      • A little confused about how much water to change. According to David Burchowitz. Editor in chief of Tropical Fish periodical. and planet Animals book on Goldfish. You should change at least 75% each week. I found from experience (unfortunately). This is true. Not talking about ammonia, nitrite, nitrate , PH etc. levels. The pollutants that build up. The pathogenic bacteria….No way test for it (so far). Yes. You can get by for a few months. Even a year with smaller changes. But a couple of years down the road. All the waste will build up. Eventually killing the fish. That’s my point. Found a solution however. Aqueon or Python water changer. just hook it up to faucet and siphon. That’s about 15-20 gallons of water per fish. Still. If your opinion differs. Appreciate response.

    • Totally not trying to be rude here but I feel like you may not have taken in the information thats being given to you. You do not have to change all of the tank water. You are only to change a percentage

    • Change no more than 1/2 the water each week. I use Seachem products: Prime Stability and Pristine each week with water changes. Goldfish are alot of work but most pets are! Use the Python- a long hose that attaches to your faucet, so you don’t have to carry buckets of water. Worth every penny!

  2. I have a question, first and foremost thank you for the info provided above. We got 4 small goldfish. And we cycled the water for one week with proper filter and since it’s cold where I am we have a heater with proper temperature 72 degrees F. It has all the decorations. I put the amount suggested of prime into the water and it was slightly cloudy on the top of the tank very slightly. It wasn’t clear water how it suppose to and now a week into having the goldfish I’m noticing the water more cloudy. I try to change a portion of the water but still cloudy. Any advice? Thank you in advance

    • Hi Liz, we hope you were able to figure out the cause by now,

      If the tank is new, it is common to get what is called a “bacterial bloom” this is what happens sometimes when a new tank is setup and beneficial bacteria grows, which can cloud the water.

      this cloudiness will often disappear on its own, but if not, check your filter, squeeze it out in a bucket of tank water, and do partial water changes until it goes away.

      It is a natural process of setting up a new tank.

      If it is not a bacterial bloom, perhaps feed the tank less, and test your water to see if there is something wrong, if there is, do water changes until it returns to normal.

      Hope this helps

  3. Add Seachem Stability and Pristine to the water. The tank has not fully cycled yet most likely. Check ammonia levels. Stability will help build the bio level in your filter. Add Stability and Pristine during weekly water changes. I usually change about 1/2 the water every 7- 10 days. I

  4. I got a goldfish yesterday and I don’t really any idea of how to take care of it any advice?

    Note and questions: bubbles have formed along the edge / top of my tank overnight, tank size is about 8 inches by about 3 1/2 inches so how often should i change the water, the tank seems cloudy but i can’t really tell, I have one goldfish that is about 3 inches long, we got some water conditioner and goldfish flakes, my fish doesn’t really seem to want to eat, he pooped overnight so should I clean the tank out now?, my mom doesn’t want to spend too much money on him so I am stuck with the conditioner and fish flakes only for now.

    If someone could please answer all the questions above that would be great thanks!

    • Goldfish are omnivores and enjoy eating small crustaceans (baby shrimp) and also as a treat, bloodworms, as well as a slow sinking goldfish pellets. It is also good to give them some blanched broccoli and deshelled peas (helps prevent bloat). Make sure you don’t leave the broccoli in there too long as it can cause the tank to become cloudy and smell bad.
      Overall, I assume you are young, but if it is possible, try and do some research on overall goldfish care. Make sure you get a filter, as well. Good luck!

  5. I’ve purchased a 60 gallon tank I plan to keep 4 fan tail goldfish once cycled ( I know the 20 gallons for the first fish and 10 gallons for each additional fish means I could keep five but with a couple of decorations I feel feel like one less fish is best) but I was wondering if I could do 2 smaller water changes a week instead of one to cut down on the amount of buckets of water I have to carry and time it would take to change 1/3 of the total water?

    • Hi Shauna, thanks for your comment,

      Of course you can split up the water changes into smaller segments if you perform regular water changes.

      some people change 5% a day 4 days a week, to lessen the work and spread it out.

      Additionally if you add live plants to your aquarium, they will help to remove nitrates and you wont need to change as much water.

      Just be sure to still test your water to make sure your levels are stable, sometimes you will find you wont need to do a water change because your water chemistry levels are still good.

  6. I have to reset a 40 gallon tank do to neglect. One small goldfish has survived. Getting it completely emptied to start over is a job. Should I advise them to switch to tropical fish?

    • Hi Jean, thanks for your comment, we hope by now the tank is setup with something new and is going well.

      It depends on the peoples situation, but generally, most trops are a lot easier to look after and aren’t as demanding as goldfish.

      If they don’t want as much maintenance, we would recommend a lightly stocked planted tank, with snails and shrimp as cleanup crew, and a couple small hardy fish.

      some really good sturdy tropical fish are harlequin rasbora, platies, emperor tetras and Danios (which can all live together too!)

      While the tank is less maintenance, they will still need care, (water changes, feeding, possibly treating if they are sick, running the correct equipment etc.)

      If someone isnt willing to do these simple things, then we wouldn’t recommend fish to them at all.
      Any pet requires lots of care and should be treated with respect as a living creature, not just a decoration.

      hope this helps

  7. How frequently do I have to wash the filter sponge if I have an internal filter, because from this article it seems I should do it weekly with every water change, where as somewhere on this website it said to do it once a year which has really confused me. Furthermore other sources online say to do it once a month. So which information is correct? Thank you in advance

    • Oh no! Definitely do not clean your filter once a year. Since goldfish are a breed that produce a lot of waste, they need their tank cleaned regularly; therefore, I would suggest cleaning the filter once a week and do a water change (25%) once or twice a week.


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