Welcome to RS
CBB fish are difficult fish to keep add to that an aggressive and established fish like a Tang and that could spell disaster for the Butterfly. I would recommend the Peppermint shrimp BUT it will take time for them to put a dent on the aiptasia problem if it's severe. These pest anemones tend to spread quickly if there's excessive amounts of dissolved nutrients, I would suggest investing in a real good skimmer to help control them.