How to Change the Water in a Fish Aquarium

Co-authored by Craig Morton

Last Updated: March 3, 2023 Approved

One of the most important things you can do to keep your fish healthy is change out the water in the aquarium on a regular basis. It is an easy job, but it requires following a few simple rules.

Method 1

Method 1 of 3:

Check the Water to Decide If It Needs Changed

1
Look to see if the water is cloudy.
2
Use a water test kit to see if the water is the correct pH level. If not, it needs to be changed.

Method 2

Method 2 of 3:

Remove Part of the Water From the Tank

You do not need to change all of the water.

1
Use an algae scraper to remove any algae from the sides of the tank before you change out the water.
2
If the filter is dirty, clean out part of the components at a time. If you clean it all at once, you will lose parts of the healthy bacteria.
3
Siphon out 10 to 25 percent of the water. Use the siphon around your gravel to remove the waste and other matter.

Method 3

Method 3 of 3:

Put in the Clean Water

1
Fill a bucket with tap water that is the same temperature as what is in the tank. Use a bucket that is only for the aquarium so that you don’t get other chemicals and contaminants in the water. If the tap water is not the right temperature, you can leave it out for a few hours at room temperature.
2
Add any necessary additives, like salt for a saltwater tank, or nitrates/potassium. Always add a water conditioner (with Stress Zyme if you need some slime coat protection) to get rid of the chloramines/chlorine. Not doing so will result in damage to fish gills and a slow death!
3
Check the pH balance of the water. If you are using straight tap water, you will need to put some neutralizer in it and let it set for a minute. Test it again before putting it into the tank.
4
Use the siphon to put the water back into the tank. This is better than pouring it in because it won’t upset the fish or the decorations.
5
Test the water again after an hour to make sure it has the right chemical balance.
6
Finished.

Community Q&A

Question

I have a 50-gallon tank with 20 small goldfish. How often do I need to run the filter?

Community Answer

All the time. Goldfish are really messy, and the smaller the tank, the quicker it gets dirty. Also, 50 gallons is very small for 20 goldfish -- you should get a bigger aquarium.
Question

How can I do a water change on a 10 gallon tank without a siphon?

Community Answer

You can use a bucket to remove some water from the tank. Then use the bucket to pour in new water to the tank.
Question

What fish goes with a cichlid?

Community Answer

Catfish are one fish type that can co-exist with cichlids. Like cichlids, catfish are extremely diverse. Choose South American catfish for a South American cichlid tank, and African catfish for an African cichlid tank to avoid disease problems.