-test water
-quarantine (I have qt'd the pair when one was sick, if they were a mated pair and I thought it would help reduce the stress of separation).
-try to diagnose and treat in qt.
Fast breathing could be a number of things. Any other physical symptoms? Infection? Coloration? Not eating? Stringy poop?
Consider the fish on it's way out. If the problem can be diagnosed and treated ASAP, you might be able to help it recover.
I always keep these meds on hand.
-maracyn I and II (bacterial infections, gran pos and gram neg)
-prazipro (internal parasites, intestinal worms)
-formalin (external parasites/fungus)
-cupramine (ich)
-metronidazole (alternative antibacterial)
+
-vitamin supplement
-garlic supplement
At this point it is key to try to diagnose, and as necessary treat with the appropriate med. Sometimes it is hard to diagnose, and you'll need to rely on a best guess for treatment.