Stain I can't identify

[cmcq]

I've got round black stains on the bottom that I can't identify and can't get rid of. I recently did the ascorbic acid treatment for metal stains. It fixed the metal stains but didn't touch these. I brushed them vigorously and they got a little white on the edges but basically look the same. They're about the size of a quarter and there are about 20 of them, mostly (but not exclusively) in the deep end which is close to some trees. Ideas? Suggestions? Thx

Charlie

[BigDave]

What kind of pool is it? Are these new stains or have they been around a while? Have you recently had episode of FC dropping to 0?