Well, I tried the magic eraser. Not much luck, with A LOT of scrubbing, some of it came off, as did some of the liner print

I also tried the vitamin c, without the sock, but just crushed some tables and sorta rubbed it on the stain, didn't seem to do anything either.

Any more ideas? Can I use some form of cleaning agent or perhaps even a diluted muratic acid to disolve or loosen the organic stain? Not sure how the acid will treat the liner, but I know it works great in plumbing, cleaning showers, etc. without harming plastic. I tried simple green too, no luck.

Is the sequestering agent a pool store product, or is there some other sources for it? What kind of maintenance program do I need to follow with it. As if I can't detect the suspended copper, how do I know when to treat again?

Thanks so much,
Clay