Try adding a few more powerheads/air bubblers to add more oxygen into the water.. Some more questions to ask..
Do your fish flash/scratch at all? Try dedicating some time at your fish tank and observe all your fish's behavior for a little while. If oxygen levels is not a problem, I suspect gill flukes. I had the exact same thing happen to my last stock. Fish breathed heavily but ate/swam just fine. Just go ahead with adding more ways to increase oxygen levels for now and keep a very close eye on your fish.