Stains are green, blotchy, smooth to the touch. I have super-chlorinated twice to 30 ppm and brushed. They were not there when I closed my pool last fall, but they were there this spring when I opened it. I assumed they were algae since the water was very green as usual. I used a Nature 2 last season but did not replace the cartridge this year after reading about it. What is the best way to tell if the stains might be from metal rather than algae?