Dissolve the mineral deposits white water leaves on glass shower doors and windows using common household acids.
Warm a cup of plain white distilled vinegar, then soak a microfiber cloth or paper towels in it and press them flat against the stained glass. The acetic acid needs contact time to break down the calcium and magnesium deposits, so leave it in place for 15–20 minutes and re-wet if it dries out.
Mix baking soda with a little water into a spreadable paste and rub it over the stains with a non-scratch sponge or an old toothbrush. The mild abrasive lifts deposits the vinegar loosened without scratching the glass. Work in small circles over stubborn spots.
Rinse thoroughly with clean warm water to remove all residue, then dry immediately with a clean microfiber cloth or squeegee. Drying right away is what prevents new spots, since hard water stains form when droplets evaporate and leave minerals behind.
Keep a squeegee in the shower and wipe the glass after each use, or apply a glass sealant (such as Rain-X) so water beads off instead of sitting and evaporating. For heavy mineral areas, a wiped-down door after every shower stops stains before they start.