It sounds to me like you are just making them retract in for a day and then they come back out the next. I'd definitely try something other than the aiptasia-x. As you'll see by the answers to your posts, most people don't use it... which doesn't necessarily mean it doesn't work, but that is generally not a good sign for a product (unless it is so new that no one has had a chance to try it).
If it is actually killing them and they are coming back really fast then I'd say, in addition to trying something different, you should really take note of how much you feed. Aiptasia, like algae problems, are often caused by high nutrient loads in a tank. Since you only have one fish at the moment, you should be careful to feed only what that fish needs and can eat in the 10-15 seconds after you feed.