Aquarium Water Change Calculator
How much water to replace to bring nitrate down to your target.
How it works
After replacing a fraction x of the water, nitrate = current × (1 − x) + tap × x. Solving for x gives x = (current − target) ÷ (current − tap). The result assumes the new water mixes instantly and that nothing else adds nitrate during the change. Big changes can shock fish through shifts in temperature and pH, so split a large change into two or three smaller ones over a few days and match the new water to the tank. Example: Lowering a 200 liter tank from 60 ppm to 20 ppm with 5 ppm tap water needs 72.73% replaced, or 145.5 liters.
FAQ
Is this calculator free?
Yes. No signup is needed and nothing you enter is stored.
Is the result exact?
Results are rounded for display. For financial, medical, or legal decisions, confirm with a professional.
Do my numbers leave my device?
No. The calculation runs in your browser.