I was at a friend's place, he has lots of black spots on bottom of his pool. He said he had cleaned them a couple weeks ago. I went and rubbed one, it is a bit hard but does flake away if you scrub it, which is what my friend did before. They are back again. Refer to picture. Just a chance that maybe people might know what this is. My first guess was black algae. But his chlorine level is managed between 3 and 4 FC.
