What ever you feed it the pieces should be no larger than its mouth. Anything raw and fish will do, and stay away from the oiley types of fish. Silversides, clam, squid, myisis, and even pellet or frozen cube food can be fed to them.
As far as frequency, depending on your lighting you may not need to feed hardly at all or you might have to every other day. If the proper lighting and tank conditions are present the need to feed is greatly reduced. However once a week is a good guidline.