Why did my fish just open his mouth wide?
The primary cause of a tropical fish appearing to breath a lot is down to there not being enough oxygen in the water, they therefore have to use their mouths to suck in oxygen rather than take it in through their gills.
Do betta fishes yawn?
The gulping you describe is totally normal betta behavior. They often gulp air at the surface and will similarly gulp underwater, though less often. Some betta enthusiasts will refer to it as yawning or something similar to a yawn.
Do aquatic animals yawn?
Q: Do marine mammals, such as whales, dolphins and seals, yawn? And if so, why? Pretty much all vertebrates yawn, says Rachel Metz, a curator at the Smithsonian’s National Zoo. Among marine mammals, pinnipeds, such as seals, do—but cetaceans, such as whales and dolphins, do not.