i like to use the pure lemon juice because IMO its as natural as its going to get with adding something to the tank. as wake said, dont use to much at once as it is an acid. if alot are present killing one or two every other day or so would prob. work well.
ive also heard of using a kalkwasser mix (you cant use to much of this @ one time either) and shooting that into the mouth of the aiptasia but again i like the lemon juice method. besides you can also use alittle of it to make some nice lemonade to sip while you do this
im like wake and wouldnt buy and peppermint shrimp and def. not a butterfly fish just to fix the problem. not all shrimp will eat it and ive had different species of butterfly pay no attention at all. a good way of putting it, why buy cats to fix your mouse problem when you'll need to buy dogs to fix your cat problem.
the way it works is that aiptasia is an anemone that grows as a pest because it will sting near by coral, most likely killing them, cover rocks, not look so pretty... anyways your basically force feeding the nem acid to its mouth ( in this case the small center opening in the center )
i would turn off all pumps, fill a needle or squirter with 100% lemon juice, get it in the tank as close to the aiptasia as possible without bumping it ( touching it will send it retracting back into the rock ) and squirting out the juice to its mouth. it may take 2 or 3 needles full but again depending tank size/amt. needed to be killled dont over do it. it may be wise to test before/after with pH. i wouldnt leave the pumps off for very long either. i would squirt one or two, enjoy that glass of lemonade, while enjoying the tank for 2-5 quick minutes and fire the pumps back up.