I am doing a green to clean on a 15k IG fiberglass coated pool and after draining it and acid washing all the tan/brown residue/staining from the walls and bottom, there are about 15 round 3" light rust looking stains remaining on the floor of the pool. I tried using diluted and straight Muriatic acid and scrubbing them, but it is not removing them. I'm pretty sure they came from the pucks sitting on the bottom of the pool, but does anyone know which product will remove them? I can take pics tomorrow and post if needed. Thanks.