I've noticed brown stains in the pool recently. I found this post (Brown stains not from iron) which recommends vitamin C tabs as one of the tests.
I assumed I need something acidic, so I use dry acid (the stuff I use to lower pH levels). Placed a bit in a sock and "painted" the pool wall, and the brown came off quite easily (only a tiny bit of pressure required)
Was it ok to use dry acid as a substitute for vitamin C tabs?
Could this be an iron stain? (they are all over the pool, bar a couple of spots).