tropical fish tank shirley aquatics

How often should I do Water Changes?

Molly 04.03.21 Aquarium

Like the majority of fishkeeping, there are lots of different opinions online about water changes. All we can do is tell you what works best for us in the form of questions regularly asked….


How often?

Little and often works best, a small amount once a week, but we all have lives outside of fishkeeping…honestly, we do! A good rule of thumb is either 5% a week, 10% a fortnight or 20% a month.


Does the number and type of fish affect this?

Oh yes! Usually, the more fish you have the more you need to change. Testing your water before and after a water change can give you an idea of whether what you have just done is enough or not.

Also, certain types of fish prefer more frequent water changes, Discus for example may only be happy with 50% water change each week.


Can I change more?

In short yes, but if you find yourself needing to do 100% water change, something has gone really wrong. The more water you change, the more careful you have to be that the temperature is the same, you use the right amount of dechlorinator and even that there is not too much of a difference in the pH between your old and new water.


Can I get away without a water change?

The general answer to this is no. Your filter will remove a lot of nasties from the water but some things will build up, not just nitrates and phosphates but tannins and even hormones. These can discolour your water and even affect the growth of your fish. You will easily see the difference in water clarity before and after a water change. Even ponds benefit from 10% each month.

There are certain types of Marine set ups that can get away without a water change, but extra care has been taken with these to keep nutrients incredibly low and replace mineral content on a constant basis using a Doser. For more information on this take a look at the Triton Method.


Can water changes help with poor water quality?

Someone more clevererer than me came up with “Dilution is the solution to pollution”! If you have anything harmful building up in your tank or pond, water changes are often the first line of defence. If you have really high ammonia for example, you should test daily and keep doing water changes until the problem subsides. You can start off with 50% water changes and then gradually reduce these as the levels drop. Water testing is once again the key to success! You can do a 50% water change and almost find the ammonia is back to the same level the following day. Part of the problem is it will work its way in to the gravel and decoration, leaching back into the water when the levels drop, the fish are also able to “relieve” themselves more freely at the lower levels. Don’t get disheartened if it seems like you are paddling upstream with this, keep at it and the levels will drop eventually.


What precautions should I take?

Chlorine is present in tap water to kill bacteria, it will do the same to your filter bacteria if you introduce it to your pond/aquarium or wash your filters with it. In the same way a swimming pool with too much chlorine stings your eyes, it will also damage the sensitive mucous membrane present on all fish…so don’t take your pet goldfish to the swimming baths with you! More importantly, always use a dechlorinator when introducing tap water to your aquarium/pond. Easylife works incredibly well for aquariums as its one of the cheapest but also one of the best, offering lots of benefits other than getting rid of the nasties in tap water. Whether you have a tropical or coldwater tank, make sure the temperature of the water you are putting in is the same as the tank water, this way you won’t shock the fish.


Is there a right and a wrong way?

Believe it or not yes, if you are changing let’s say 5 buckets worth of water. Firstly, make sure you have a bucket that’s dedicated to the fish tank and not used for soaking Dads pants in. Secondly, take all 5 buckets worth out first, and then replace them slowly. Don’t do one bucket out one bucket in, this is a much less effective way as you end up inadvertently taking some of the new water back out again and instead of doing a 20% water change you have probably only done a 10% change. Also, for Tropical fish tanks, use a Gravel Syphon. This way you are removing the dirtiest water from the tank and cleaning the gravel at the same time.


Anything else?

Water changes are often a good opportunity to add back minerals, so if you have a cichlid tank, or live in a soft water area, try and make a point of adding back some of the minerals during a water change, that way they will get distributed more evenly. Test the pH and ideally kH of your tap water so you know exactly what you are putting back in.


Are you sure that’s it?

Don’t always assume that your tap water is better than the water in your aquarium. There are a number of parts of the country where nitrate and phosphate levels in tap water are incredibly high. Not dangerous to us, but you might find you are only adding to the levels of organics in your tank when you do a water change. If this is the case, then a water purifier or buying purified water is the best course of action.

Back to blog


tropical fish tank shirley aquatics

Molly 04.03.21

How often should I do Water Changes?
Read More
sunny pink pond lily

Molly 04.03.21

How to look after Pond Lilies?
Read More
Probiotic Koi Fish Food

Molly 11.08.20

What is the best food to keep my Koi and Pond Fish healthy?
Read More


Martin Litt 17.03.21

Pond Plants – Shirley Aquatic’s Quick Guide
Read More

Molly 26.09.18

How do I feed frozen bloodworm to a fish?
Read More

Molly 26.09.18

What is the best thermostat for my Heat bulb?
Read More

Why buy from us?

  • Great Customer Service
  • The personal touch
  • We enjoy going the extra mile
  • Always on the lookout for new and exciting things
  • If we haven’t got it we will hunt it down and get it for you
  • Competitive prices
  • Exclusive ranges
  • Decades of experience
  • Great relationships with all of our suppliers, they are as keen as we are to help you
  • Thousands of products on display
  • Knowledgeable staff with experts in every aspect
  • Last but not least…We care! Sounds cheesy, but we care about you, your pets, our store, our animals and each other
Read More Reasons to buy from us.