GENERAL INFO :
The gill or gills of a betta may become inflamed, because of nitrate poisoning, and possibly bacterial infections, or even a defective gill
One or both gills will not close completely. Gills may look red on the inside (inflamed) or not, your betta may be gasping for air, unable to breath and ultimately die.
Isolate sick betta. Do a full jar water change every second day. Every time you change water, add Ampicillin to the water. In the case of nitrate poisoning, simply add one drop of methylene blue in betta’s tank.