and since it was mentioned that you have clams and featherdusters,I just want to say,you won't have them anymore once a cbb is added.
I wouldn't put one in a 55 either,they definately don't like crowding in.
Clams? IIRC an occasional CBB might nip at them, but I believe thats the exception not the rule. If nothing else my CBB never got the memo about munching clams....