I had my prize Male Ps. Aurora get spooked and hit the glass at full speed one day while cleaning the tank. He lost the use of the rear of his body from that day on. He lived for several years afterwards but I had to keep him in a less competetive of an enviroment. He still would spawn with my females too.
Couldn't tell you if this is the deal with your red, hard to diagnose.
Keep in on a diet with spirulina, if it has minor blockage in it's digestive tract it can put pressure on the swim bladder which might keep it on the bottom. Spirulinw digest easily and can help with blockage.
Wish I had a solid answer for you Ash, THis is a tuff one......
Joel