You need muriatic acid (with caution).

Yes, that’s the standard fix. Calcium deposits on pool tile are essentially scale — calcium carbonate that bonded to the surface. Scrubbing alone won’t cut it because the bond is chemical.

Muriatic acid is the go-to because it dissolves calcium. But you have to be careful: it’s strong stuff. Wear gloves, goggles, and old clothes. Dilute it with water (usually a 1:10 ratio, acid to water — always add acid to water, not the other way around). Apply with a brush or sprayer, let it sit for a few seconds, then scrub and rinse thoroughly. Don’t let it dry on the tile, and never use it on metal or stone coping.

If that sounds too aggressive (or you have a fiberglass pool), there are softer alternatives like a pumice stone or a specialty calcium remover from the pool store. Those take more elbow grease but won’t wreck your finish. Just avoid anything abrasive on glass tile.

Test a small area first, because you’re about to learn something about your tile’s condition either way.