For a SPS reef, mixed reef, or LPS and clams, the Peppermint Shrimp is still the best answer in my opinion. Not a fish as you asked but the highest success rate with the minimum amount of risk. Just make sure you get the right species as they are often mis-labled. Also get more than one, as some individual pep's are more hungry for aiptasia than others. After the last pest is gone, keep them in your tank for another month or so (may have to feed them) and you will never see another aiptasia in your tank again.