Need help with sick Jack Dempsey

I have a sick Jack Dempsey on my hands. The fish has been ill for almost a
month. To be honest, I would have written sooner, but I've been expecting
it to die and it hasn't.

The fish can't seem to straighten up, it's back is constantly crooked to one
side and it just floats at the bottom of the tank most of the time. It can
swim around in loops for short periods and tear up the tank, but it always
settles back down at the bottom of the tank after a short while.

The fish exhibits no external signs of disease, injury, or infection other
than being bent into a constant left turn. It can't or won't straighten
out, and it hasn't eaten in a very long time.

It is the largest of my Jack Dempseys, and there is no sign that the other
fish have ganged up on it. The other fish in the tank are all perfectly
healthy, active, and eating normally. The water condition is excellent as
usual.

Any ideas on what is wrong with my largest Jack Dempsey and how to treat it
would be greatly appreciated. Thank you.