A couple years ago I noticed a stain in my pool. I tried the typical things to remove it, brushing, chlorine, etc. I finally decided that a piece of asphalt shingle had blown into the pool and had dissolved while I was away and it was stained with tar and moved on.
Since then it has gotten worse and now I know it is not a stain, but something more dreadful is happening. The dark area is now an area that ~1/4 inch deep pitted area. When I brush it clouds of what look like rust are removed. I've looked at it closely underwater and it is just a really dark pitted mess.
This year I got a second one a couple feet away from the original one. I'm now wondering if somehow the water has penetrated the concrete and is rusting out the rebar, causing the area to flake away and become pitted? I really don't know and am hoping that someone here may have a better idea. I've attached a picture to help.