If it is only a single or few aiptasia, you could try the Kalkwasser paste trick on them. I bought a cheap biocube on Craiglist that had some corals I wanted and decided to keep the cube as a QT. I soon noticed an aiptasia showed up, so I blasted it with some Kalkwasser paste and it died quickly. Another showed up 3 or 4 months later and I blasted it too. I have not seen any others yet, but it is a quick an easy fix if you only have a few to deal with and you don't want to treat the entire tank.