Aquarium water change calculator
Enter a reading you want to bring down (usually nitrate), your target and your tap water's reading, all in the same unit (usually ppm, which is the same as mg/L). The calculator gives the water change that gets you there, in one go or in smaller steps.
Result
Change 50% (50 litres)
| Plan | Each change | Water used in total |
|---|---|---|
| One change | 50% (50 L) | 50% (50 L) |
| 2 changes | 29% (29 L) | 59% (59 L) |
| 3 changes | 21% (21 L) | 62% (62 L) |
How the calculation works
A water change swaps part of the tank water for fresh water. Change 30% and 30% of the nitrate goes with it, replaced by whatever is in your tap water. The calculator works backwards from your target, and also shows how to get there in two or three smaller changes.
Before a large water change
- Match the temperature of the new water to the tank.
- Treat for chlorine and chloramine with a dechlorinator, following the label.
- Go gently after a long gap. If the tank hasn't had a change in weeks, its pH and hardness may have drifted from your tap water. Spread the work over a few smaller changes on separate days.
- Test again afterwards to see where you landed. See the aquarium maintenance checklist.
Not sure of your tank's volume? Use the aquarium volume calculator. For what the readings mean, see aquarium water parameters and how often to change aquarium water.
Common questions
How much water should I change to lower nitrate?
Changing a share of the water removes the same share of the nitrate, if your tap water has none. A 50% change takes 40 ppm to 20 ppm. If your tap water has nitrate, you need a bigger change, and you can never go below the tap water's level.
Is one big water change or several small ones better?
Several smaller changes are gentler on fish because temperature, pH and hardness shift less each time, but they use more water overall. Taking nitrate from 40 to 20 ppm needs one 50% change, or two changes of about 29% each.
Does this work for ammonia and nitrite too?
Yes. Ammonia and nitrite should be 0, so in an emergency aim as low as you can and test again after each change. Treat new water with a dechlorinator that also handles chloramine.
How often should I change aquarium water?
For most tanks, 10–25% once a week keeps nitrate under about 20–40 ppm. Your own nitrate tests tell you whether you need more or less.
More tools
Keep your tank's tests, water changes and costs together in AquaLog.