You don't want any kind of large crab in with fish. Do you have a refugium or big sump you could put him in?
What kind of suggestions are you looking for... Invert ones? Porcelain crabs are filter-feeders that are harmless to everything but copepods.
You could try a flame angel, perhaps...
How about a watchman goby/pistol shrimp pair?
Out of curiosity, how is your copperband butterfly doing? How long have you had it?