I finally realized the couple dark spots in the pool (that had been there since we bought the house last year) were black algae. It really didn't look a lot like the pictures here and never grew...so I didn't think thats what it was.
It appears to have etched itself into the finish a bit, but after over a week of brushing with a metal brush, it appears to be gone.
When brushing the sides with the metal brush, I got some cloudiness that would come up in the area I brushed. But it went away pretty quickly. Though I have some staining, I never saw anything I would call calcium scaling, the aluminum ladder looks normal...
But now by CH is up to 425...up 100 from a month ago. Confirmed over separate tests about a week apart. I believe my testing procedure was correct, as I've had consistent results over the past year until now.
FC: 3.5 (I had not added today's Chlorine before the test)
CYA from a couple weeks ago was about 55, nothing has been added that should have changed that.
Borates are about 50
Salt was at 2300ppm back in May..this is NOT a salt water pool. I suspect this is all from liquid Chlorine or perhaps salt added by the previous owner. I haven't added any.
Water temp stays in the mid to upper 80s. Full sun all day until I get my sail-shade installed this week.