How to use the aquarium water change calculator
- Enter your tank volume.
- Enter the percentage you want to change.
- Press Calculate.
Worked example
A 200 litre tank where you want to change 25% of the water.
- Tank volume
- 200 L
- Water change (%)
- 25
- Tank volume
- 200 L (52.8 US gal)
- Water change
- 25%
- Water left in tank
- 150 L (39.6 US gal)
Arithmetic only. Choose the percentage for your fish from species advice or your aquatic vet; treat new water as you would for any change.
Example values are illustrative; this result is produced by the same calculator above.
How we calculate it
Sources: Pure arithmetic / unit conversion on user inputs (no external constant)
Understanding your result
A water change replaces a share of the tank's water with fresh water. The calculator multiplies the tank volume by the percentage you choose: for a 25% change of a 200 litre tank, 200 × 0.25 = 50 litres.
It also shows the same figure in US gallons and the amount of old water left in the tank, so you can plan how many buckets to fill.
This tool does the arithmetic only. How much water to change and how often depends on your fish, stocking, filtration and water quality, so follow advice for your species or ask your aquatic vet or fish shop.
Common mistakes
- Using the full-tank volume when decor and gravel mean there is less water in the tank.
- Entering gallons while the unit toggle is set to litres, or the reverse.
- Treating a percentage as a fixed rule for every tank.
- Adding untreated tap water directly. Follow the water-treatment advice for your fish.
- Changing far more water than planned because the bucket size was never checked.
Good to know
- Matching the temperature of new water to the tank reduces stress on fish.
- Test kits show how your tank is doing and help you decide whether to change more or less.
- Use the Aquarium Volume Calculator if you do not know your tank's volume.
Frequently asked questions
How do I calculate a 25% water change?
Multiply the tank volume by 0.25. For a 200 litre tank that is 50 litres.
Which volume should I use?
Use the real water volume if you know it. Our Aquarium Volume Calculator gives the full-tank figure, which is slightly higher than the water actually in the tank.
How much water should I change?
That depends on your fish, stocking and filtration. This calculator only does the arithmetic, so follow advice for your species or ask your aquatic vet or shop.
Can I enter the volume in gallons?
Yes. Switch the unit toggle to imperial and enter US gallons. The result is shown in litres and US gallons.
Does this tell me how often to change water?
No. How much and how often depends on your fish, stocking and filtration, so follow advice for your species.
What volume should I enter if the tank is full of decor?
Enter your best estimate of the real water volume. The Aquarium Volume Calculator gives the full-tank figure, which is a little higher.
Sources
- Pure arithmetic / unit conversion on user inputs (no external constant)
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Estimates only. Every pet is different — confirm with your veterinarian.