the best thing is a varied diet. other then live fish they will take a liking to all sorts of fresh fish from your grocery store. they will love fresh salmon, smelts, haddock, and shrimp!
whenever smelt was in season, i could feed my eight 6-7" rbp's for a couple weeks for 2 bucks!
also remember that these fish grow fast... in a year they can easily reach 6"s. just make sure you got the room for them!
good luck!