you dont need to crank up the temperature. Leave it fishless for 6-8 weeks. You can leave the snails and crabs in there, they do not host the ich parasite or any other fish parasite.
Sorry, I read that you put the clowns in 2 weeks ago and assumed they brought ich in to your tank. I would QT 4 weeks minimum, 6 weeks prefferable before adding to display.