Most butterflies are notorious for not being "aquarium" safe. Yes, there are definately success stories out there, but in my experience, far more death stories than success ones.. talking 90% death with most butterflies in an aquarium. To top that off, your putting it in a 40G tank? For a Klein's butterfly it lists Minimum Tank Size: 55 gallons. I would definately think 100G+ is much better than 50G for that fish. While the Klein's is one of the better ones to chose from, much better than a copperbanded, it is still a touchy type of fish that really needs its needs met for success.
Also, you have a maroon clown in the same tank.. definately a very aggressive fish, which on top of putting a butterfly in a tank too small for it which causes severe stress to the fish, it is more than likely also getting harrassed by the clown. Double whammy almost guaranteeing death to any butterfly in that tank.
Angels on the other hand.. while almost as bad as Tangs when it comes to ich, are usually hardier than butterflies depending on the type of angel (Dwarf I hope). Still, angels can also be touchy fish to get established, but once they do they usually last a long time. You may want to consider changing out the maroon clown for one a little less agressive like an ocellaris. Otherwise your going to have a hard time finding acceptable "friends" to put in the same tank.