Peppermint Shrimp! I had an large infestation of Aiptasia and purchased two peppermint shrimp from my LFS. In less than two weeks, I could not find a single speciman of Aiptasia anywhere in the tank, and the Peppermint Shrimp had changed from Clear with Red-Stripes to Aiptasia colored with Red Stripes.
Just make sure you get real peppermint shrimp and not Camel Shrimp.