There is no way to treat a fish while it is the DT. If one fish has ich, its only a matter of time before other fish show signs.
The ONLY way to treat ich is to pull all the fish out, let the tank go fallow for 6-9 weeks and treat the fish in QT with copper or hyposalinity. Now, that might not be an option for some people... do you what you can to eliminate stress on the fish and make sure he keeps eating. You can try garlic to help stimulate appetite.
Now the ich cysts may disappear from the fish, but the fish still has it and every fish from now on will be susceptible.