I get metal stains at the same spots in my fiberglass pool every few weeks. When I apply ascorbic acid, the stains disappear completely and the surface looks perfectly white again but when I run my hand over the area I can feel some roughness. What happened was the pool was put in last fall, winter/snow came before electrical was hooked up, the pool was filled with water, there were lots of leaves and dirts in it. The PB covered the pool with a large tarp. In the spring the PB was able to get the swamp back to crystal clear without changing the water. Before the tarp was put on I remember I used my hand to wipe away some stains(didn't know what kind of stain that was at that time) at one area on the pool wall, so now the area that I wiped stays stain free but right next to it the metal stains always start to form because of the roughness, I tried scrubing with a brush but didn't help. Other than this minor issue my pool is stunning with sparkling water.