Water-change impact

What does a 30% water change actually do to your nitrate? Or to your KH? Mass balance, in one screen.

Stormy & Manfred · the watchers

Stormy reads where the tank is now. Manfred shows where it lands after the change. Same formula the in-app forecast uses for every scheduled water change.

Current valueWhatever you're tracking — NO₃ in mg/L, KH in °dKH, etc. Stay consistent across all three fields.Tap valueSame parameter, measured straight from the source water (or 0 if it's a parameter your tap is effectively free of).Water changePercentage of tank volume replaced.

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.

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