Stormy & Manfred · the watchers
Water-change impact
What does a 30% water change actually do to your nitrate? Or to your KH? Mass balance, in one screen.
Enter current, tap, and the water-change percentage to see the result.
How this works
A water change replaces a fraction of the volume with tap water. The replaced fraction takes on the tap concentration; the rest keeps the current concentration. Adding the two contributions weighted by volume gives the post-WC value.
result = current × (1 − wc%) + tap × wc%Caveat: this assumes the tank is perfectly mixed and that the parameter doesn't react during the change (true for nitrate, KH, GH; less true for CO₂ or chlorine, which off-gas or react). Use it as a planning estimate, not a guaranteed result.
In the app, Manfred watches the WC cadence you set and walks the same math forward week by week so you can see when the tank will drift out of range before it does.