Could be a few things but if the damsel is the only fish showing symptoms it could be caused by the aggression of other tankmates. What other fish are in the 55 gal
and do you have a QT if needed?
Other than that, I would suspect low ph
or possibley this is a newer fish that was either hurt during capture/transport or may not have been acclimated properly.
Any other possible signs other than rapid breathing? Odd swimming patterns, poor appetite, color or skin afflictions?