Improper water chemistry & neglect. It depends upon how severe the deposits are. The best way to remove calcium from your pool tile is, to sand blast it off with glass bead. Surface Renewal does this.
Calcium build up is almost invariably a result of excessively hard water. In addition to this calcium, other chemicals that are added to the water will further cause the calcium to precipitate out of the water and to adhere to all the surfaces.
