I wrote about this a few weeks ago thinking it was a 1 time problem - it's recurring.
I have 2 female bettas in a divided 5 gallon tank at work. The tank is heated (to 78) but not filtered. I change 25% of the water every other day and about once a month a 100% change. They seem to be very happy.
Lately one of them becomes bloated after every time she eats. It doesn't look TOO bad, but she floats. Not on her side or belly-up, but if she dives to the bottom of the tank she'll get pulled up to the surface.
I've tried isolating her in an epsom salt solution (approximately 1 tablespoon per gallon). The bloat will go away, she'll swim normally, and then the next time I feed her it happens again. I've fasted her for up to 3 days while in epsom salt doing 100% water changes, but I'm afraid to fast her for much longer than that. This has been going on for about 2 1/2 weeks now. I'm not sure what the problem is.
I feed them bettamin tropical medley flakes 2x a day, a very small pinch each time. The other betta has had NO problems like this and is still eating and swimming normally.
For what it's worth, the sick betta still has great color.