Using Metal Free-any tips?

[poolbee]

Thanks. I think because of the process I'll wait until fall before closing to do this. It's been so hot here (and everywhere) that we"re in the pool every day.
My 12 y.o. had a grand time the other day, outfitted in his snorkle gear, using the vit. C to get the spots off the liner. He was amazed that scrubbing with the MC Magic Eraser couldn't do it but a few rubs with the vit. C did. He was a cleaning fool! Knock yourself out I say!
Anyway, I am hoping we can stay ahead of the stains with the vit.C until fall.

I do not think it is my water, but that the pool builder used Morton's table salt for the SWG. He put in 550 lbs of it. They were in 50 lb. brown paper bags. I remember seeing the word "restaurant" on the bags so it didn't concern me. It said that it was 99.6 or so pure which I thought was fine.
I have learned otherwise;that the anti-caking used in it has iron in it.
So, I don't think it will be a recurring problem. I am going to take my tap water to the pool store to have it tested for iron to be sure. My water authority claims 100ug/liter-not sure how that equates to ppm.

Thanks again for the info.
One big question-since I have a vinyl pool and can't drain it, is there even a point to using the Metal Free? I had been under the impression that it would be filtered out.