How should I remove hard water scale from pool tile — with acid or a pumice stone?

For heavy, stubborn scale, use muriatic acid diluted 1:10 with water — wet the tile, apply with a sponge or sprayer, let sit a minute, scrub with a nylon brush, and rinse well. For light, powdery scale, skip the acid and use a wet pumice stone (real volcanic kind) rubbed gently on the tile. Test pumice on an inconspicuous spot first, as it can scratch softer tile like travertine. For pool plaster or fiberglass, don’t use acid — try a phosphate-based cleaner or mild vinegar soak instead.