My tank is 225litres, the crabs are small (1 inch) x3 Mithrax, x2 Halloween Hermits, probably about 7 small other selection of hermits. x3 shirmps who are all quite big at 2 1/2 inches each.
The butterfly was new, from lfs, within 2 days covered in Marine Ich. ( I also bought a really small baby tang at the same time, who also had/got the ich, but he is still going about quite happily)
Due to UV steriliser, hasn't affected anyone else, (at least not yet, but it's been nearly 4 weeks so hopefully it won't either) Potentially he could have had another reason to go, but we will not know now I expect! Butterfly was about 3 Inches from tip of nose to tail end.
I have looked in the tank this morning and one of the shrimps has shed its shell, so maybe this would indicate that he has been feasting? (they shed their shells quite regularily, so maybe completely unrelated, but I don't believe in co-incidence where my tank is concerned.!)
As for the Ich, I'm hopeful that the UV will prevent its spread, I've got a higher value of Reef/invert than fish, so we shall see but fingers crossed so far.