In order to kill off all of the ich, you need to get ALL of the fish out of the display tank and leave it fishless for 6 weeks. During that time, you should treat all of the fish in the q-tank for ich, whether they show signs or not. The only two proven methods is copper treatment or hyposalinity treatment. Copper is quicker, but I feel that it is more dangerous to most fish. Hypo, IMO, is safer if done correctly, but you need to have a refractometer and it takes on about 6-8 weeks to treat using this method. A short hyposalinity dip isn't very helpful unless the fish are peppered with ich. In that case, I'd to a straight freshwater dip for 3 minutes.
Here's a link on both the copper and hyposalinity treatment methods.
ATJ's Marine Aquarium Site - Reference - Treatments