I have a brownish stain on the bottom of my pool around the drain. I have tried the ascorbic acid treatment with little to no lifting. I tried the wet/dry sandpaper with no change. We tried scrubbing it with the chlorine puck which didn't work, however, leaving the puck for a few hours on the stain is working. Is this safe for my plaster? The stain is fairly large. Is there a better way to do this than placing several pucks on the stain and moving them around until the stain is gone?