A PB in a 55 is pretty questionable. i would have to say nil. I have a funny story about PB's though. My girl and i went to the Denver zoo, and they have an aquarium set up (not sure on the gallons) but they had two PB's in it and all they ever did was chase each other around a huge piece of staghorn coral. I think they liked each other cause they didnt do it in a harmful way, just a sort of playful, courtship ritual, but i am not kiddign i laughed my @$$ off, cause they would chase one way for about a minute, then stop and go the other way. It was sooo funny. and they are a truly beautiful fish!
good luck
jon