It happens inevitably to every reefer. Soak frozen mysis in garlic or buy the pellets that are coated in garlic. Garlic has been shown, anecdotally at least, to boost fish's immunity to fight off the ich. Clowns are usually more resilient than say tangs so the fact that they have it could be a sign of a pretty wide infestation.
The only real long-term solution is to set up a QT tank, remove all of the fish from the display to that QT tank, and either use hyposalinity (low salinity) or copper based additives to kill the ich. You must leave the fish outside the display for 3-4 weeks to allow the remaining ich to die off in the display (w/o hosts (fish) in the display they eventually die off).
If I were you I would try the garlic for a few days and see if it helps. Others will probably suggest an immediate removal to a QT tank.