Most likely a swim bladder issue. Do they have problems staying upright/straight? If they do have an injured swim bladder, when it heals (If it does) they will return to the rest of the fish.
Ah ha! I had exactly the same thing - and then ended up feeling very very silly when I posted on here!!
Apparently - what you and I take for fish that are desperately trying to stop themselves floating to the top, is actually fish trying to impress possible mates with their bobbing little dance.
As a test to make sure.. drop a tiny bit of food into the tank and see if their disease 'miraculously' disappears! (It worked for me)
you could soak the flakes for a bit in a separate container so that they sink when you put them in instead of floating. that will help reduce the intake of air.
Off topic kind of, but one of my neons has a bent tail kind of. I dont know how it happened. I have four neons and three of them always swim together and play but the one with the bent tail is always under the cave. Will his tail end up straightaning?