FishMaths

Water change calculator

Enter your nitrate test result and your target. You'll get the size of water change that gets you there.

Test your tap water once. Many cities have 0 to 10 ppm.

Result
0
Enter your numbers

How the math works

A water change removes a slice of old water and replaces it with tap water. The percent you need is (current minus target) divided by (current minus tap). Splitting it into two smaller changes needs less total water swapped per change, which is gentler on fish.

What nitrate level should I aim for?
Many keepers aim for under 20 to 40 ppm in freshwater tanks. Sensitive fish and shrimp do better on the lower end.
Is a big water change dangerous?
Large sudden changes can shock fish if temperature or pH differ a lot. Match the new water's temperature, use conditioner, and split very large changes across a few days.