Aquarium Water Change Calculator
Work out how much water to take out and put back, how many buckets that is, what a nitrate reading will roughly be after the change, and how much water conditioner your product label calls for. Everything runs in your browser: nothing is sent anywhere and nothing is stored.
Not sure? Use the volume calculator first (net volume, after substrate and decor). Enter the label's amount and the water volume it treats, in the unit chosen above. The calculator only scales the label dose to your fresh water volume; always follow the label.Enter your tank volume and press Calculate.
How it is calculated
- Volume to change: tank volume × percentage ÷ 100.
- Buckets: volume to change ÷ bucket size, rounded up to full buckets.
- Nitrate after the change: assumes the water is fully mixed. New nitrate = old × (1 − p) + fresh-water nitrate × p, where p is the fraction changed. Water held in filters, substrate and plumbing mixes imperfectly, so a real reading can differ. Test to confirm.
- Conditioner: label dose ÷ treated volume × volume of fresh water you add.
Reference: nitrate after one change
Starting from 40 ppm in the tank and 0 ppm in the fresh water, ideal mixing gives:
| Change | Nitrate after |
|---|---|
| 10% | 36 ppm |
| 25% | 30 ppm |
| 50% | 20 ppm |
This page is a general calculator, not advice about the care of any particular animal or plant. Check how much and how often to change water for your livestock with a reliable specialist source.