I bought a house with a pool last year, so I am a little new to all of this. The pool had been closed all of last year and it was a green mess. Got it all cleared up thanks to lots of bleach and the water looks clear. I had a couple light stains in 2 of the bottom corners. I tried scrubbing them but to no avail.
I shocked it and they got lighter, but started get darker again. I shocked again and scrubbed with the brush. Again they got lighter, but came back. I looked around online and thought maybe it was a metal stain, so I got some vitamin C tablets. I put them in a stocking and rubbed them on the stain. Did nothing (it may have gotten slightly lighter, but that could be from rubbing it or just imagined). I used about 20 tablets. Now, the stains have now gotten darker and now light stains are forming all around the bottom. Help!
Vinyl liner, inground pool. ~18,000 gallons.
A few pics of the stain: