He's in a 2.5 gallon, heated, unfiltered tank. Water changes are done twice weekly (1 50% and 1 100%). I have one of those Marineland tanks with a full glass hood, but I keep the level of water low enough for him to come up for air. He lives by himself, has some plants and a rock cave that he likes. I've had him for about a month now, and he was probably a few months old when I got him. He's a VT, and his tail isn't fully grown yet.
I use the Omega betta pellets, and the expiration date isn't for a while. I can try to change foods as soon as I get out to a store, which might not be for a few days (college student whose also an athlete). My room generally I'd say is cooler than the tank temp, but not by much. I always have the heat on and a window cracked slightly so the air isn't too dry. And the water temp is always right around 78*-79* (My heater is set for 81*).
I don't have any good pictures of his stomach, but if it happens again I'll be sure to try to get some.