Over time swimming pool tiles can accumulate a white line from the calcium hardness in the pool water. Sometimes, this can be removed by hand rubbing at it with tile cleaning blocks and in other cases, it might be too bad and shall need removing via a process called “Bead blasting”. Basically, very small grains of sand is blasted at high pressure to the tile which cleans off the calcium hardness water lines to expose the tile’s original color. It’s best to drain the pool when having this process done so that the water can be refreshed and the hardness level within it reduced. If the water has recently been drained and refilled then it could be done without draining it. In these cases, the water would just be lowered to expose the tile so it can be cleaned.