Pool water looks good, but I have some sort of black stuff on the bottom and it appears to be spreading. I've brushed it several times but it doesn't change/move at all. I've brought the pool up to shock levels, but didn't maintain them there as I didn't have time to do overnight FC loss. The CC's go to 0 though. Should I go back and repeat the shock treatment? Anything else look to be a problem from the pictures?
Thanks for any help,
Pool water looks clean: