I just pulled the cover off this morning and found a huge black stain in the deep end. The pool is a 20' x 40' gunite with about 36,000 gallons. The black stain is about 6' in diameter in the deep end of the pool. I tried brushing it with a small metal bristle brush and it came up slightly, but not significantly. The water is clear and there's a few misc leaves and such which made their way under the cover, but there's not a ton of debris in there.
I'm thinking maybe a copper stain from the winterizer - I used two Winter Pills (Amazon.com : AquaPill - WinterPill 4 Garden) same as prior years - but have never seen this issue before.
Any ideas what it might be?