Have you tried live brine shrimp? A cockle meat on the half shell might work, too.
I assume you didn't quarantine the new tang. There is a very good chance that the "Ich" will spread to all the fish. Quarantining new fish is good because you avoid infecting your main tank and other fish with disease and parasites but also, it is much easier to get a single fish feeding when it is in a tank on its own.
It is likely that if you don't treat the two tangs they will die from the "Ich". You may need to treat the clowns, too, to ensure they don't carry parasites. I recommend hyposalinity, but it needs to be performed in a separate treatment tank. If you treat all the fish and leave the main tank without fish, the "Ich" will die off after around 4 weeks.
See:
Marine "Ich" and
Hyposalinity.