Trapped air from floating food?
If it's exclusively at eating time.
(Feed slow sinking pellet and/or bloodworm, or push the flake into the water body so they don't eat from the surface)
Second easiest thing to check, water hardness and adverse effects of continued hyper osmoregulation. (Neons being a softer water species)
Then explore disease........both parasitic and bacterial "disease" can affect swim bladder function.