I used a "magic eraser" on the tile at the beginning of the season and it cleaned the tile well. Now it's back. I teach swim lessons so the pool is heavily used during the week. It's kind of a yellow looking scum. The magic eraser has worked well on the top of the tile, but there is still some at the water line that I can't get off. Two questions 1) what can I use to get it off? 2) Is there something I need to be adding to the the water to prevent this?