What is the best way to remove iron stains from a plaster pool?

Ascorbic acid (vitamin C powder) works better than anything else for iron stains on plaster. Sprinkle it directly on the stain in a test spot; if it disappears in a minute, you’ve got iron. For a full pool, lower chlorine to zero, broadcast a few pounds of ascorbic acid, and circulate for 24 hours. The downside is it eats chlorine and can mess with pH, so you’ll need to shock hard afterward and use a metal sequestrant to prevent stains from returning.