Thanks for all of the info! I'm still a little confused in this particular case, though...
He's pooping, and it's not white. So probably not parasites. He has the appetite of a shark. He just always has a big belly. I don't feed freeze dried food, just frozen occasionally.
Curses, my camera is dead. Well, this is a more recent video, and you can see his belly in it. This was after fasting for at least two days, as well.
Could it be because he's a young fish? He's only 4-6 months old. Do younger fish get bigger bellies?