Well water and metals
Wells frequently carry iron, sometimes manganese, occasionally copper. In the ground it is invisible; the moment it meets chlorine in an oxygenated pool it can drop out as a rust, brown, or purple-black stain.
The mistake is shocking a freshly filled pool hard before sequestering the metals. Chlorine first, sequestrant second is the wrong order — with well water it should be the other way around.
Well water can also be high in hardness, high in alkalinity, or unusually low in one of them. Test before the fill, not after.
Softened water is not the safe option
A water softener strips calcium and replaces it with sodium. Filling a pool with fully softened water gives you water with almost no calcium hardness — and calcium-hungry water attacks plaster, grout, and tile.
If you fill with softened water, calcium has to be added deliberately to bring hardness into range. Skipping that step is one of the fastest ways to etch a new surface.
Softened water also loads sodium, which matters if you ever plan to convert to salt or already run one.
How to fill either one safely
Test the source water for hardness, alkalinity, pH, iron, and copper before you start.
For well or metal-bearing water, run a hose filter and add a sequestrant early in the fill.
Build hardness and alkalinity deliberately to hit an LSI-satisfied number rather than assuming the fill water got you there.
Add stabilizer last and modestly. It is much easier to add CYA than to remove it.
