I just posted on another thread this trick that works GREAT!
Get a suringe and boil some RO water. Fill the suringe.
Now when you do a water change save the old water and submerge the rock in the bucket quickly so the aiptasia fans out and you can see it.
Pull it out of the saltwater fast and blast him in the center with the suringe of boiling water.
They will literally melt off the rock.
Rinse the rock in the old saltwater in case some of the dissolved aiptasia is still on the rock. Then immediately put it back in your display tank.
If you just hit the spot where the aiptasia is, you can keep most of the rock's beneficial bacteria intact.
Hope that helps you...
Denise M.