I have stains, I believe, as well in a relatively new pool finished in Sept. last. I initially saw one long 'stain' that I thought was on the finish. Through winter, pool became 'green', and I have shocked that out. But now there are a few darker spots that do not brush out.
Yesterday, I turned the brush over, rubbed on the spot, and looked like a tar like substance, dark in color. Is this scale, or algae