Water Change Calculator
Water changes dilute: swapping 25% of your water removes 25% of whatever is dissolved in it. Enter your current reading and your target to see the change you need.
How to use the result
The source water field matters more than most people expect: if your tap water carries 10 ppm nitrate, a water change can never get the tank below 10 ppm, and every change toward a nearby target has to be bigger than the simple dilution math suggests. Test your tap or RO water once and reuse that number here.
Avoid changing more than about 50% at once unless it's an emergency: large swings in temperature and chemistry stress fish more than an elevated but stable reading does. Split big corrections over consecutive days. For ammonia or nitrite above 1 ppm, act the same day — those are toxic, not merely undesirable.