Yah...make sure the tank is completely and fully closed. If you do that for any fish, this would be it!
Also, what size tank is this? Or are they in separate tanks? Usually, they will fight with each other or other surface dwelling fish so they are best kept singly.
Try feeding yours frozen whole baby krill. I chop them in half to make them more manageable and my butterfly seems to enjoy getting a really big meal now and then. He can eat almost 8 full krill.