I have a crayfish I just caught in a stream, and at first it would try to get out, but once I gave it plenty of hiding spots and fed it enough, it seems to be pretty fine. I don't have a lid on my 55g, and I had 3 in there at one point. Not a single one got out. The best hiding spot I have for mine is the corner of a cinder block that is buried in sand so it can dig its way under. It really likes it there. As for fish, I only lost tiger barbs when I had it in a 30g with them. Right now I have a JD, male convict, 5 bleeding heart tetras, 2 plecos, and 2 striped raphael cats in my 55, and the crayfish never really seems to go after them. Maybe blue crayfish will act differently, who knows. I have lots of plants in my tank as well, so the fish could avoid the crayfish pretty easily.