OK Hi - new here and new-to-pools last summer and battling "The Stain" ever since.
27kgal IG plaster w/stains on walls and some on floor.
Jack's stain kit tells me metals.
I brought C down to 0-.5, pH to about 7 on Sunday and put in, through the course of a few 1/2 applications as recommended, a total of 5lbs ascorbic acid and (3) 32oz bottles of metal seq. and 1-1/2 32oz bottle of 60% Poly Quat.
The few dark (almost black) stains I had have disappeared but the brownish stain all over the sides and bottom hasn't really changed at all.
15min puck-in-a-sock with scrubbing afterward doesn't change stains.
I've added some pics of "The Stain". The third one shows it the best (in sunlight) and also so streaking that has always been there; please excuse the concrete/coping/tile - that was to be fixed "this year" after the stain was fixed "last year"....