Hey I would differ slighly, there is no point moving them, the tank is already infected with ich and it will only stress them further.
You should gradually increase the temp to about 28C and treat with a white spot cure like King British WS3, the dose should be half to 2/3 of that stated because clowns do not have scales like most other fish and so the medicine can get into their system more easily.
I am currently having an out break with ich. The other fish did get a bit but they are much more resilient and most have fgot rifd of it, however the clowns still have it (two in paticular). Some times ich lingers for a while because the medicine has to get it when it is free swimming in the water/gravel. Increasing the temp to about 28c drastically speeds up the life cycle of ich from weeks to a couple of days.
That is your best bet. Clowns are prpbably the most susceptible fish to ich in the hoobby.