Muriatic acid is your best bet for pool tile scale.

Yes, but dilute it (typically 1 part acid to 10 parts water) and work in small sections with a stiff nylon brush. Never let it dry on the tile—rinse immediately. For light deposits, a pumice stone (wet it first, scrub gently) works without chemicals. Avoid metal scrapers or wire brushes unless you want scratches.

Wear gloves, goggles, and old clothes. The fumes are no joke. Test a hidden spot first. If you have a special finish like pebble or quartz, check with the manufacturer—acid eats soft surfaces.

For stubborn scale, a commercial scale remover (like Scale Free or similar) is safer and works just as well. Pricey, but less scary than mixing acid yourself.

You do not want to learn about muriatic acid the way I did.