Clownfish in my experience are somewhat slow to acclimate to new tanks regardless of their characteristic hardiness. They gradually start up with new foods, but will do so in their own time. This unlike Angels and Butterflys that often if they don't start eating aggresively on the first day, never will.
Frozen brine seem the be the best 'starter' foods for clowns and most other fish. Other aquarists seem to swear by mysis shrimp, but I find the frozen variety too large for smaller clowns, so I keep the Sally brand always around.
One of my new angels started attacking freeze dried brine shrimp floating on the surface 30minutes after being released into one of my tanks. Now that's a healthy fish.