Moved into a house a month ago here in Texas. Pool (6 years old, 15000 gal, gunite, SWG (not functional yet), Polaris vacuum) had green algae at the time of move in. Had a pool company get it shocked and cleared up and I've been maintaining the chemicals since and using the TF100 kit.
Yesterday my pH was 7.5, FC 2, CC <0.5, CYA 60. I added bleach yesterday to get back up to 7.
Since we've owned the pool, I've never had CC go above 0.5 or have my FC drop less than 2, but I've recently noticed these black splotches on the bottom of the deep end of the pool. They're flat and don't seem to scrap off. Measure about 1-2 inches each.
I'm obviously concerned about black algae, but everything I've read makes it sound like that would be a little "prouder" on the surface and maybe smaller in size. These seem to be more like a stain (see photo below).
Any help would be greatly appreciated.
PS - Since I'm a newbie to pool ownership, and while researching this problem, I realized that I should be brushing my pool regularly. Hadn't been aware of that. Any recommendations on a brush to purchase, and how frequently should I brush the surface?