I have purple on the walls of my pool. We have tested for algae, copper, metals etc. I have the pool drained because of tile and coping issues (a different post) and the purple is stubborn and so I'm not sure how to get rid of it (we did acid wash before but may have to try again) and to prevent it from coming back. We are replacing the sand in our filter. We have a 20 x 40 gunite pool, sand filter. Water is perfect and beautiful (til we drained it). The purple is really purple and not blue at all. I will try and take a picture. No pool places can figure it out either...anyone else ever have this issue?