Marta

Other way round. Distilled has nothing in it at all and makes hollow, thin coffee, because there is nothing there to pull flavour out of the grounds. You want moderate minerals and low carbonate. Distilled is only ever used as a base that people then add minerals back into.

Alkalinity and hardness are two different things and this is where most people get stuck, because in tap water they usually turn up together.

Hardness is calcium and magnesium, and you want some of it. It is what carries flavour out of the grounds. Alkalinity is carbonate, and it buffers acid. A cup can be perfectly extracted and still taste dead because the carbonate neutralised the acidity on the way out.

That is why "my coffee is flat" almost never responds to grinding finer. You are not under extracting. The acid is being cancelled after the fact.

Not a nitpick, that is the exact trap. A standard softener swaps calcium for sodium and leaves the carbonate completely untouched. So you lose the useful half and keep the harmful half. Softened water is usually worse for coffee than the hard water going into it.

Look for bicarbonate on the label rather than just "mineral content". Under about 50mg/l is a sensible starting point. Most of the very cheap still waters are fine. The expensive ones are often the worst, because high mineral content is a selling point when you are drinking it.