So Bela's got this bubble of air in his cruet. It got stuck in there when I took the jar complex out to rinse it and then put it back in. So I have seen Bela in there sipping air from the bubble. And what I wonder is this: if he keeps breathing in that air, what's going to happen to it? -- because he doesn't precisely exhale. So is the bubble just slowly going to shrink? That's what has to happen, right? He takes a sip of air and swims away and the gases dissolve in the water. That's assuming that the air he breathes exits via his gills, though. I'd guess that's the case, but I'm not sure. I will have to ask the Internet. I suppose it's possible that he's exhaling on the sly.
If he is exhaling, and the air bubble doesn't actually shrink but is slowly replaced by whatever it is bettas exhale, I should tip the cruet up to get rid of it. I don't actually trust Bela to notice if he gets a mouthful of carbon dioxide instead of a mouthful of oxygenated air; he might suffocate through pure stupidity.