Neon tetra gasping at surface?

We put 6 neons into our new tank on 3 days ago and the other 5 have been feeding fine but one is now just floating near the surface all the time gasping. I lloked up osmotic shock but the tank has plenty of oxygen and i'm testing regularly. Can i do anything to save it? I don't have another tank to set up a "hospital/quarantine" for the poor thing. No other fish in tank, only 2 tiny snails