My little female betta died today. She'd been acting oddly for a couple of days, not her usual perky self, and I asked for advice on the betta forum, and the only person to respond suggested a water change and "fasting" her for a couple of days (not feeding her). Well, I'm not going to starve one of my babies, and she wasn't eating much, anyway. I had just changed her water on Saturday. I change everybody's water once a week. She was acting different before I did that, so it wasn't the water change at fault (sometimes a water change can affect a fish badly if you don't take care to equalize the temperature and properly treat city water before putting the fish in it). Both the boys are fine and I did everything the same for all three.
She was lying on her side but still alive when I got ready to leave for church rehearsal this evening, so I scooped her out and put her in a bowl with some betta medicine and she didn't even argue much about being scooped, which she usually hates. When I got home, she was gone. I looked her over and couldn't see a thing wrong with her. No swelling. No discoloration. No injury, where she might have bumped herself somehow. I wish I knew what had gone wrong, so I could be sure to avoid it with the boys.