This Large Cleaning Block is suitable to remove tough hard water stains, mildew stains, algae, lime rust, and other mineral build-up with the Large Cleaning Block for swimming pools. You'll love this environmentally friendly and chemical free way to keep your pool clean and clear of debris. It's pumice-like ceramic tile cleaner will not scratch the surface of our pool and easily cleans tile, grout, gunite, plaster, and concrete surfaces. Simply scrub with a circular motion, using light pressure, above or below the waterline to remove stains, dirt, and debris.