I'd definitely agree that it's probably all do to stress, and to get more floating plants. It also probably wouldn't hurt to turn the light down on the tank if ya can to help reduce stress. Also I would suggest adding another 2 butterflies to the tank to help disperse aggression in the tank, from the way it looks to me the one is stressed because it has nowhere to hide from the other fish in the tank so any and all aggression is going directly to the one fish instead of being dispersed thru multiple fish.
Btw I seen your mention of guppy grass in here, which would be a great floating plant to keep in the tank, another good floating plant to keep would be frog bit or dwarf water lettuce.