Our pool was replastered in March and we had a new pool light installed. We have noticed a dark black-ish stain under the light. PB (that installed light) said that our 'old harness' leaked or something like that. Then he said he would give us a bottle of metal out and for us do the following:
1. Turn off pool, 2. Dissolve metal out in water and, 3. slowly pour right over the light area.

He said we would see a cloud and it would pull the metal stain right out of the plaster. Do we want to do this? What do you recommend? Love this site! Joyce