I have a few black spots showing up on my top step. They are a raised round spot, looks black, and if rubbed, I can see it smear green. Is this black algae? It doesn't seem to look like pictures I've seen others post. Just a few raised 1/4 inch diameter spots here and there. I don't think I've ever had anything but FC = TC and no CC, but yesterday noticed a slight bit of red tint when testing for CC. My FC got low and I didn't notice right away.
FC 1.5 - usually kept around 4-5
CC barely .5
I don't remember CA right off hand, but think it was 260 or so.
I was going to raise the CYA back up to 80 (we've had a lot of rain and a lot of water has gone out the overflow), and then set the pool up with borates. But figure I need to tackle these black spots first. Shocked last night with 6% bleach to about 17 FC. I didn't think about leaving it high for a couple days, so it's probably dropped some today in the sun while I'm at work. Will scrub the spots with a wooden brush tonight. I guess I need to keep the FC high for a couple days, brush the spots, and see if they go away.
How high do you think I should bring the FC?
21000 gal, in ground, salt Aqualogic PS8