if its only breathing out of one gill, then gill flukes is most likely. You can move it into a hospital tank and treat with formalin of 2 drops per gallon, redosed every other day, right after a 50% water change. Do this for atleast 2-3 weeks to be sure to kill not only the active flukes, but also to kill off any of the newly hatched flukes since their eggs are not affected by the formalin. Never hurts to treat the whole community tank if it is bare bottom to make sure there arent any more hiding.
-Ryan