when ur yellow boxfish is stressed or dead. it will release toxins into the water that will kill off everyother fish relatively quickly. this is why boxfish are not recommended for the reef aquarium. they will also nibble on tubeworms . shld also be kept in a min 125 gallon tank.
ur butterfly.. what species of butterfly is it? Its going to eat your corals and probably ur cleaner shrimp when it gets bigger.
They are not abnormally prone to white spot. its juz inevitable .