Descale-It is a bio-friendly chemical. It works by scraping off the big objects first, then spray on the pool scale to remove the rest.
This cleaner removes scale rings, films, and crusts from the surfaces of swimming pools and spas. It's safe to use without harsh chemicals and doesn't affect the pH of the water.
Leslie's Descale It 1 qt
Descale-It removes scale rings, crust, and films from the glazed tile, metal, acrylic, and fiberglass surfaces common to swimming pools and spas