I agree with the guys here, ph shock seems to be the most obvious thing that might have caused this. However as wailua boy said there can be a number of other possibilities. You said your jar is 5", at this size aros are super sensitive. I'd stop feeding feeders so you can rule that out. Hopefully it didn't already contract a parasite, though that's what it seems if the fish is eating and the stomach stays sunken and he hasn't grown. As for the back snapping back and forth in and out of place, only thing I could think of would be him ramming into something, maybe when he goes into shock. I have seen fish dislocate their jaws and they have to snap it back in place all the time, not sure how long they survived, the spine is a more serious bone however, I wouldn't see one doing something like that but I guess anything can be possible.