Vitamin C does the trick. Rubbing tablets on the stains just above the current water line got them all off, but the stains at and just below the water line and on the fiberglass steps remain. So it looks like I should try an ascorbic acid treatment.

Question. My fc is down to 0, ta is about 100 and CYA is 65, but my PH is 8.2 Should I lower the Ph a bit before using the ascorbic acid? Any advice about about the best sequestering agent to use?