Ascorbic acid is the best for iron stains in plaster pools.

Ascorbic acid (vitamin C powder) works better than anything else for iron stains on plaster.

Just sprinkle it directly on the stain in a small test spot. If it disappears in a minute, you’ve got iron. For a full pool, you’ll need to lower chlorine to zero first, then broadcast a few pounds of ascorbic acid (follow the label). Circulate for 24 hours. The stain lifts, and you vacuum up the iron particles.

Downside: ascorbic acid eats chlorine. You’ll need to shock hard afterward and maybe run a metal sequestrant to keep the iron from coming back. Also, it can mess with your pH.

But honestly, this is the fix. Cheaper than a full acid wash, and way less aggressive on plaster. Just don’t skip the sequestrant afterward or the stains will return within weeks.