I have a 15,000 gallon cement pool. Opened and cleaned up from the winter. Water is clear but we have stains on the walls and floor. We have used metal out 2 qts with no results.never in 15 yrs have we had this problem. It's a DE filter system and works great. I'm at a loss on what to do. Pool store just wants keep trying to sell me more "stuff". I super chlorinated and no results. Anybody have amy suggestions on how to remove these stains without draining and doing a acid bath???