Does ascorbic acid work for most metal pool stains?

Yes, a vitamin C treatment (ascorbic acid) is the standard fix for iron and copper stains on pool plaster or vinyl. Lower the pH to around 7.0, then broadcast ascorbic acid over the stain area — it reacts quickly, and the stain fades or disappears within minutes. For stubborn copper stains, you might need to follow up with a stain-removing sequestrant like Metal-Free.