I think a good combination for a 55 aggressive tank would be a fu manchu lion, valintini puffer, and snowflake eel, just be sure that the puffer and lion are large enough not to be swallowed by the eel. Include plenty of hiding places. Live rock would be my choice. Not only does live rock provide cover for your fish, it also acts as a biological filter. If you have not chosen a substrate yet, I would use a deep sand bed. Again an excellent biological filter. good luck...
Almost forgot... if you do go with the eel, be sure that the aquarium lid is escape-proof, if there is a way for the eel to get out, it will.