coral hind grouper with ich

I've recently started a new tank and used base rock to keep parasites and hitch hikers down to a minimum. Well I felt the need to seed my tank with purple coroline and picked up a rock from Petco. My new grouper has ich I believe. My buddy had a new bottle of herbtana that I could use, so I'm giving that a try for now. Has anyone had luck with this before. He's been scratching himself for 3 days now. How long does this last and could the rock from Petco have had the parasite in/on it? Will this kill my fish?