Last Friday I noticed a black spot on the bottom of my pool. It's about 3/4 inch in diameter. I've had black algae at the water line and in some poor circulation areas before, and it's clearly not algae. My pool water is crystal clear. I rubbed it with Vitamin C and it did nothing. I tried scrubbing the spot with a green Scotch brand scrubbing pad and it did nothing, except it seemed to scrape some of my plaster off. I can feel an indentation from the scrubbing. I didn't think the scrubbing pad was that abrasive to do that. The only thing I can think of is that an imperfection in the plaster was somehow revealed--perhaps by adding MA. This is around the area in the deep end (over a return) where I usually add MA.
I will try to post a picture.
My latest test results are:
CL 9.0 (I'm lowering it to around 4-5)
Last week I lowered my pH a little too much to 6.8 (I was lowering TA from 90) which caused my CSI to be about -.7 but it was at 7.2 (CSI -.38) a day later and a couple of days later it rose to 7.8 where I expect it to slowly increase to around 8.0 at which time I will lower the pH back down to around 7.5. I wonder if the 24-36 hours at such low pH could have caused the spot to appear or whether its from adding too much MA in one spot--my pool is only 4.5 feet deep in the deep end.