Remove metal stains from a plaster in-ground pool

[waterbear]

Chlorine and oxalic acid also destroy each other and oxalic acid is the ingredient in Pool Stain Treat from United Chemical, which is designed to be added to the pool water just like ascorbic acid, in addition to the Pool Stain Treat spotting bags..
The oxalic acid will reduce (make soluable) metal stains, particularly iron and manganese, while the sufamaic acid is going to remove scale (calcium).

FWIW, I have seen ascorbic acid cloud a pool. It is temporary. Ascorbic acid works on all pool surfaces, it just doesn't work on all types of metal stains. It is great on Iron and manganese and may work on some copper stains but usually does not.