I'm also doing the once-a-week fast and pea regimen with my fish now. Bettas are carnivores, I've learned, but the dried and pre-packaged diet you get in the pet store (most convenient so I go with that for now) can swell in their stomachs and give them constipation. That's when they have the trouble swimming, don't float normally or sink on the bottom. The remedy is to skip meals one day a week and the following day feed them a quarter of a green pea (cooked, cooled and skinned) which helps them "go" again. Supposedly this keeps them in better condition. So I'm trying it today.
Some redundancy here; but I'm writing this mostly to remind myself of what to do to care for my little pets properly.