Welcome to the forum.
I received a VM from you and I think you may have contacted or replied to me by mistake-especially since I only use natural methods to treat Betta and rarely if ever recommend antibiotic for treatment, with that said, I am not critical of those that do-its a personal choice...Anyway, you may need to check and see who asked that question and reply in their VM.
Your betta has bacterial or fungal disease; possibly cotton wool, columnaris, gill disease that can block gills and and prevent breathing. Treat with a broad-spectrum antibiotic that treats fungal diseases, like Mardel's Maracin Plus for 5 days. If that does not work, try Maracin Copper Safe for 5 days, take a few days off between med cycles to avoid immunity; treat on off days with a combination of API Stress Zyme plus Kordon AmQuel; then move back on to Mardel Maracin Maroxy for 5 days max. Add 1 tsp per gallon of Aquarium Salt, ne--never regular salt to discourage bacteria and fungi. Get a Hydor heater for aquariums less than 5 gallons. Remove uneaten food; feed 5 pellets every other day; never overfeed; avoid methylene blue for serious contagious diseases. See fish diseases; other posts; keep water temp constant; always add good water conditioners like Stress Zyme,Kordon AmQuel, plus Aqua Salt for each weekly water change; never keep bettas in less than 1/2 gallon H20.