Keeping mullet in an aquarium?

Hi,
I have a 200 gallon marine aquarium currently holding 4 juvenile (10-15cm length) flathead mullets. Does anyone here have experience keeping mullet in an aquarium? In particular, I'm curious what people have been able to feed them, and how. Although I've seen the mullet feeding on the algae in my aquarium, I haven't as yet managed to get them to eat anything I give them even though I've tried pellets, flake food, nori, and frozen brine shrimp.

Thanks for your help!

P.S. Although I agree that 200 gallons is not going to be enough space for 4 fully grown mullet, please consider that I will be releasing them back into the bay from which they were caught before they reach such a large size.

Thanks for all the responses! The mullet so far are in great condition and they are feeding pretty extensively on the algae growing in the tank. Chompers - what size were the mullet you had and could the filter feeding behavior have been because they were still young? I think I will try live brine shrimp as a supplement and see how that goes.