Originally Posted by LittleBettaFish
Looks like a stressed/sick female betta to me. She may just be very low on the totem pole and because there is such a small number of females in the sorority there is not enough fish to spread the aggression out.
Also, did you acclimatise her properly before putting her in the new tank? She might be clamping her fins because of the change in environment. The tank itself looks rather bare. Bettas tend to love densely planted tanks because it gives them the security of having somewhere to hide if they get stressed out. Large, leafy plants are a great favourite with my bettas.
Since you have taken out this female, I would keep very close watch on the other two that have been left behind. Usually, if kept in pairs, the dominant female will often harass the more submissive one and possibly injure it.
There were 3 in the tank originally, but a bit ago I added a fourth so it was a sorority of 4. Now that this one has been removed, there are 3, and they seem to be doing really well in their tank.
The tank shown in the pictures is my small hospital tank that hasn't been used up until now except when quarantining the new fourth fish. Its ornamented with a large ruin like building to swim through and a cave in the back. I'm planning on getting a plant or two as well, but I don't see how you could consider this a "bad" tank.
When I introduced her into the hospital tank, I cupped her with the same water from the big tank into a bag, acclamated her for 20 minutes, added some water in from the new tank, left it for 20 minutes, then poured her into the hospital tank. So it can't be that. Its been over two days now and she's still acting like this.