Acid removed part of stain, advice please

[RANDY M]

Hello to everyone,

FC - 9
CC - 0
TC - 9
Ph - 7.8
TA - 180
CH - 600
CYA - 70

Pictures 1 & 2 are from 7/2010. The 3rd is from today when I used straight acid with the Jandy stain remover. I had to keep it in place for a few minutes while it siphoned about 1/2 gallon of acid onto it. It seems to have worked when pucks didn't. Does this mean it's a mineral stain, not black algae? Is there something I could use to treat the whole pool at once? Any advice would be appreciated. Thanks.

 

[257WbyMag]

Is this a stain or is it a deposit of some sort? Is it rough or smooth?

 

[JasonLion]

That looks kind of like black algae to me. Black algae will partially scrape off with a finger nail and when smeared on paper looks green. A metal stain will not brush or scrape off at all.

 

[RANDY M]

These have been here for a couple of years now. I originally thought they came from ground water because there was never any debris left on the the pool floor. They don't seem to get any bigger. I tried scrapping it off with my finger nail a while back and and got nothing. Not one of my best ideas but it does scape off with a chisel duct taped to a pole, so it seems to be just on the surface of the plaster. Zero results with rubbing a puck on it or liquid chlorine in the Jandy stain remover.