Eels will take out small inverts and unsuspecting fish, even a juvenile specimen. I used to keep a couple reefs that was more predator oriented, and it was quite fun for awhile. You just have to be careful what you stock.
http://www.youtube.com/watch?v=uFiRTB4Alwc
http://www.youtube.com/watch?v=m0Y65EojRa0