We make glass tile for pools - for above and below the waterline. Mineral deposits found in water will leave a mark on any material, whether it be glass or porcelain. Pool salt blasting with Kieserite will remove hard deposits on any type of tile and will not harm glass - Oceanside Glass has a product called Calcium Release which is a ph-neutral cleanser. I've used it on my hard water deposits on my shower door with great results. The bottom line is to clean the pool regularly for optimal results. The custom glass fountain pictured was made using our glass tile and is located in southern California. I would also suggest looking into using Oceanside glass tile, Sicis glass tile or Bisazza glass tile - all make glass tile mosaics and small squares/rectangles that work well in wet conditions such as pools and fountains.