Breathing heavily could be nitrate poisoning ... test the water ASAP. That's how I think I lost one of my Green Tiger Barbs. It stopped eating and was breathing rapidly, but all other fish in the tank were ok ... my nitrates at the time were nearing the upper 40ppm range.
If your T-Barb that's hiding out by the filter appears ok and will eat, then it's a
probably a T-Barb behavior I see with one of mine. Could be another on the T-Barbs challenged him for dominance and he's hiding out. Also do not be surprised if they start to point their bodies with their heads towards the bottom, tail towards the surface.