I put one in my 50 and saw it 2 times in 4 months. They are small, nocturnal, and very well camouflaged. Really cool little crabs, but very difficult to keep up with. They are excellent little hiders! I transferred everything from my 50 to my 90 when I upgraded, but I've never seen the little guy in the big tank. He may be in there, maybe not. I'll probably never see him in there though, even if he did make the move....