I don't know what the rest of you find, but IME any new fish added to a planted tank with CO2 tends to gasp until it acclimatises.

I've been stocking my newly set up tank slowly and every time, the newly added fishes head to the surface. It's not usually until the next day that they recover. Most LFSs seem to keep them in tanks with with little greenery and airstones running. I now try to add them late at night with the CO2 and lights off. By morning they're fine.