Is there a type of fish you can keep with your pet blue crayfish?

if its a blue moon crayfish your talking about, then no. they're territorial and aggressive, and they would attack and eat any fish you put in there. however, if it was, say, a loach, then the loach would eat the crayfish. if you had Mexican dwarf orange crayfish though, they can be housed with any type of fish that will not eat them. they're very calm and docile.