I have an odd stain problem I have been unable to rectify. I'm not even sure if "stain" in the correct terminology as this seems to be more of a deposit. I have small, the size of a head of a pin, reddish-brown spots on the vinyl liner and fiberglass stairs of my pool. In many cases I can pick them off with my thumb nail but not always. I have tried Jack's Magic Stain ID kit with no substantial positive results. I have tried citric acid in a sock as well as vitamin C tabs with some, very minor improvement. A chlorine puck does nothing. PH Down (dry acid) does seem to help, especially on the fiberglass stairs.
Here are my numbers from last season