Black-ish stain under pool light..

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

You can just get a bottle of vitamin C tablets, crush them and put them in a sock - then rub it on the stain. Vitamin C is ascorbic acid - it will take the metal stain off. I would try this first, let me know if the vitamin C does'nt work.

Thanks Marie. Will try the Vitamin C this week.