I have exactly the same problem. I had some staining when I inherited the pool with a new house. I got a lot more last year when I switched to Baquacil. I learned too late that a metal problem may occur with Bacquacil, and I'm assuming it is metal, but I don't know. The gunite seems pitted at the stain, as it is rough to the touch. I don't have a test kit yet, so can't give numbers. I did have an algae problem, which has cleared up with Bleach treatment. Now I only have the yellow stains, and a few rust marks left from nails I found in the pool when I bought the house.

Any help in removing either would be greatly appreciated.

Thanks!