I have a sorority of 7, and they're in a 55 gallon tank with neon tetras, zebra danios, julii cories, and oto catfish. But one of the girls is always off by herself and she's so much smaller than the other girls. I know bettas are not "social" fish in general, but mine seem to be. They're almost always together in the same area of the tank (or maybe that's only when I'm home because they hang out in the same area where I always feed them lol), and I'll only see a ripped/nipped fin once in a while. But I have one girl who stays by herself mostly. Actually a lot of times I can't find her. The thing that worries me, though, is that she doesn't seem to have grown at all. I've had one of the girls for about 8 months, another for about 4 months, and then I got the remaining 5 the same day about 2 months ago. They've all grown a lot since then except for this little one (she's only about the size of my neon tetras). She also never comes up at feeding time. Some of the girls get annoyed by the other girls taking all the pellets and a lot of times they will instead go eat the sinking pellets I put in for the cories. But I don't think she does. So she's really skinny and hasn't grown at all, and now her caudal fin got pretty nipped up last week.
Is she sick, or should I consider putting her in her own tank? Maybe she feels too intimidated by the other girls, or is just not as social as they are and therefore gets nervous being around them?
Anyone else have an experience like this with a sorority?