I had something similar happen a few years ago with a Firemouth. Same exact thing, from normal and healthy to unable to move anything behind his pectoral fins. I tried everything under the sun thinking it might have been swim bladder disease and nothing worked. When he stopped eating I was going to euthanize him but decided to try Melafix, which I always had thought of fish snake oil nonsense. A day after I added he was back upright and eating and about a week later he regained his strength, color, and some movement in his tail. I was shocked. Unfortunately he died about 6 months later but he got back to being himself for that last little bounce back period. The Melafix definitely could have been a coincidence with his natural healing but ever since I'm a believer.
The best explanation I found was possible genetic condition from inbreeding at commercial fish operations, but really who knows?
The best explanation I found was possible genetic condition from inbreeding at commercial fish operations, but really who knows?