A variety of foods will end up showing you the best results. Aside from their planktonic needs, its good to feed them larger meaty foods as well.Shrimp/fish/squid/just about anything that is 'fresh seafood' will do just fine.
I feed silversides and frozen shrimp to mine.
A example of feeding silversides -
Take a full silverside out of the freezer. Thaw it by running warm water over it for a few seconds. Cut the silverside into small chunks (quarter inch pieces usually work well). Then just sprinkle the chunks over top of the anemone and it will grab them and pull them into its mouth.
Feed them about every 3-4 days.