I've had good luck with injecting the boiling hot lime juice in them. The trick is to stab it way down into their body cavity and then push the plunger. More natural and cheaper than the Ap X, although I do like the syringe that came with that product.

My favorite tip came from Erik H. If the aptasia is on a rock that you can pull out of the tank, remove it and torch it with one of those candle clickers. Completely effective and highly satisfying!