Once a tank has ich it will never, never be gone. also any fish that has been cured of ich will be a carrier of the disease and may infect any new tank mates.
Thats not entirely true, there is a way to rid your tank of ALL water born parasites including ICH by adding a UV steriliser to the filtration system. I would recomend a UV steriliser to anyone who doesnt mind spending a little extra cash to prevent diseases in the first place rather than spending the cash later down the line on medications. I run UV's on my two larger tanks and have never had a case of ICH or any other disease, the UV also prevents the spread of algea.