[naesco]

The reality is this is an extremely difficult, as pointed out by other posters. but not impossible fish to keep.
It simply dies without any apparent reason just as the OPs did and IMO should not be available in stores for sale.
It should be available to reefers who order them in. That way fish loss will be much much lower for this species.
It follows than that it should not be recommended for dealing with aiptasia problems when there are other critters and methods available that do not sacrifice a fish.

I also agree with the comments that the very few who are presently having success keeping them have an obligation to post their failures with the species to provide reefers with a balanced view.