Tokis, thanks very much for this. I think this was the problem. The molly was so jittery I couldn't even catch him to put him in our bully tank. But the bullying gradually eased over the following week, and the molly gradually straightened out. He's fine now.
Our black marbled sailfin molly has suddenly twisted his back :crazy: It was quite sudden. He was fine until another molly chased him in circles - then he came out wonky. Since then he has been stuck in an S shape :huh: He is feeding and swimming quite well. But is there anything we can do to...