I am new to this site but looks like lots of very well informed advice! I have a gunnite pool that I had built about 5 years ago. It is in just north of NYC so has to be closed each fall. Usually there are leaves that fall onto the cover during the winter and some get under the cover. There has always been a stain each year but our pool maintenance guy has always put something in each year that has caused them to miraculously go away. This year he has tried a number of things and nothing much has worked. He advises that the stains are different this year and are due to the early and heave snowfall at the end of October last year that he believes pushed more water out of the pool than planned and resulted in the leaf debris sitting on the gunnite in some places. He has tried Jacks Blue and lately brought me a canister of ascorbic acid to try but that seems to be having only a limited effect? Here is a picture! Any thoughts, guidance would be greatly appreciated!