So this is not a fun first post to write, but...
I have a 10 gallon moderately planted tank that originally housed 6 female bettas. No one was being bullied and everything was pretty settled, but I noticed one of my girls, Red, was becoming very thin despite continuing to eat. This went on for a couple weeks and then she became weak and unable to swim properly. At this point I moved her to a very shallow container and tried antibiotics. After 2 days she was still hanging on, but I knew she wasn't going to make it and couldn't even swim up for air so I had to put her down. I was very upset, but everyone else looked okay.
Then I noticed one of the other girls, Yellow, was becoming very thin and swimming a little erratically. In the last 3 days she had actually been a lot better. Great appetite and better control. Then tonight, about 20 mins after feeding them, I look up to see that she's being swept around by the currant, completely rigid and breathing heavily. I took her out of the tank in a plastic cup where she rested sideways at the bottom and spasmed a few times and died with her head arched back. The whole thing was pretty horrifying and I have no idea what happened!
I did introduce some amano shrimp to the tank about an hour before, but everyone else seems fine, so I don't think it has anything to do with them. Could she have over eaten? I'm worried about my remaining 4 girls and completely clueless as to what is caused the wasting. 3 of the 4 are still fat and happy looking and the one that isn't has always been a bit small. Anyone have any ideas?