Peppermint shrimp, longnose raccoon or copperband butterfly. I bought a CBB local since it was plump and eating frozen and while I haven't seen him doing it all the aiptasia are gone from visible surfaces - they will do a number on feather dusters too - oh yeah and yesterday he started picking on my lovely open brain so I threw that in my nano.

Peppermints are hit or miss - make sure you get the tropical atlantic species Lysmata wurdemanni instead of the pacific one. I would get at least 4 or 5 too in an average tank - will take them a while to find them but once they do they will eat all but the very large aiptasia which you will still have to kalk/boilingwater inject.