How often should you change the water in a tropical fish tank?

Fish pollute water as they breathe and defecate, but they are not adapted to live in their own waste. The only way us fishkeepers get away with keeping fish in aquariums is by way of biological filtration, and water changes. Without these fish would be poisoned by the water they live in. 

Filters colonise with beneficial bacteria which convert toxic ammonia excreted by fish first into nitrite (also toxic,) then into nitrate. The easiest way to deal with nitrate is to remove it via a water change. Nitrate is constantly building up in your aquarium (because its a byproduct of biological filtration,) so if you remove a percentage of tank water with new water (with a lower level of nitrate,) the nitrate level will come down. 

Change water frequently and although tapwater contains some nitrate, it’s usually lower than the level in the tank. Test your tapwater and tank water for nitrate. Aim to keep tank water at 40 parts per million or less. To do this your tapwater will need to be below 40ppm, and ideally below 10ppm. If the tapwater nitrate level is high, consider conducting water changes with Reverse Osmosis water instead, which is purified.

Water should also be changed to lower phosphate levels, remove tannins and staining, and to buffer pH and KH. Fish tend to grow faster in tanks which are water-changed regularly, so a water change may also be removing growth-inhibiting hormones, which don’t build up in rivers in the wild.

So the frequency of water changes should be based around the nitrate level in the water. A low nitrate level and technically your water shouldn’t need changing, although a change does benefit in the other ways also listed above. 

Never remove all of the tank water in a water change, and little and often is best. Set yourself a regime and stick to it. Remember that filter media should only be cleaned in old tank water so water change time is the perfect time to combine filter cleaning. The mucky water can then be used to water your plants. 

Start with a routine of a 25% water change every two weeks. If that keeps the nitrate level low, stick to the regime or you can even replace slightly less. If nitrate levels still climb, you need to change more water at a time, like 30%, or more frequently, like weekly. 

If you dechlorinate properly and always bring water to the same temperature, there is actually no limit on how often you can change the water. Professional fish breeders may change water daily in order to remove excess food and encourage maximum growth. 

But if you don’t change the water often enough nitrate level will rise, pH and KH will drop, and you’ll get something called Old Tank Syndrome, whereby pH drops so low the biological filter stops working properly, your hardy fish just about survive it but newly purchased fish die within hours or days of being added. 

How to do a water change

To do a water change you will need a syphon tube and a bucket. You aren’t meant to start the syphon by sucking the pipe for hygiene reasons, so instead look for one which can be started by hand. Place one end of the tube in the tank, the other end in the bucket on the floor, start the syphon and you are away. Keep an eye on the tank end to ensure no small fish are sucked down the pipe, and the bucket end to make sure it doesn’t overflow. 

Discard the dirty tank water and take the bucket to the sink. Fill with cold water, a little bit of warm water and check the temperature to make sure its the same as in the tank. Add liquid dechlorinator as directed, swirl it around by hand and then leave for a minute. Pour the water back into the tank and your water change is done. If the water removed causes the pump to run dry, turn the pump off before the water change and back on after. 

Water change with gravel clean

Buy an aquarium syphon tube and it will almost always come with a wide bore gravel cleaning attachment. Gravel vacs are great because you get the opportunity to clean your gravel while also conducting a water change. Uneaten food and fish poo builds up in aquarium gravel and can harbour nitrate, so its best removed. Choose a gravel cleaner which suits your size of tank. Too small and it will take a long time to remove water. Too large and it may not fit in under the tank lid, may struggle to start syphoning, and it will remove water too quickly, emptying the tank. 

Other ways to do a water change

In small nano tanks, you could simply use a one-litre measuring jug. You know its volume and it’s very quick and easy. In huge tanks, you could literally bail water out using a bucket, although its hard physical work.

If you don’t want to put your back out you could simply pump water from the tank. Connect an aquarium powerhead or pump to a long hose and put the other end of the hose out in the garden or down a drain. Just make sure you don’t forget and pump all the water out, or that an unprotected pump intake doesn’t suck in your fish. 

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Jeremy Gay is an author and freelance aquatic specialist. A former editor of Practical Fishkeeping magazine, he offers a wealth of experience on all things aquarium and pond.


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