How water change calculator Works
The dilution formula
Change % = (current − target) ÷ (current − tap). The tap-nitrate term matters: 5 ppm tap water caps achievable dilution, and well-water at 20+ ppm means water changes alone cannot win — RO blending or heavy planting becomes the strategy. The calculator flags that ceiling explicitly.
Frequent-and-smaller beats rare-and-huge
Two 25% changes beat one 50%: smaller swings in temperature, pH, and chemistry stress fish less while removing the same nitrate load weekly. The stable-tank philosophy is gentle dilution on schedule, not periodic floods — and the weekly test tells you when the schedule needs adjusting.
The refill discipline
Temperature within 2°F (shock is a real fish killer), dechlorinated (chlorine burns gills; chloramine needs the right conditioner), and refilled gently (a saucer on the substrate beats a waterfall). The mechanics are boring; the boring is why the fish are alive.