What is the best way to remove calcium scale from pool tile?

Muriatic acid (diluted hydrochloric acid) is the best bet — it’s cheap, fast, and works better than most off-the-shelf scale removers. Wear gloves and goggles, dilute it per instructions (usually 1:10 with water), apply with a stiff brush or sprayer, let it sit for a minute, then scrub and rinse thoroughly. Don’t let it dry on the tile or mix it with bleach, as that creates toxic gas.