I have diamond brite on my pool and I have a yellow stain at different locations on my pool floor. the stain is not yellow algae because it will not brush away. The surface of the pool is where the stains are is pitted and rough on your feet. I have tried crushed chlorine tablets in a sock on the stains and nothing happens. I have also tried vitamin c in a sock and nothing happens. Does anyone have any idea as to what the problem is and how to correct it. Thank you in advance for any help.