Angelfish with very severe equilibrium problem

Hi the other day I bought some quarter size angelfish and 1 of them has
a very severe equilibrium problem. I mean he can lie on the ground and
every time he tries to swim he ends up crashing into things and turning
upside down. He cannot really move. I have him a ten gal quarantine. I
put him into a 1 quart container with the lid with 500 mg of human
amoxicillin from a capsule within the tank for a 1 hour soak. It was the
only antibiotic on hand. The only other stuff I have is quick cure.
First what can cause this. I read in a book chilling can cause it. The
Angelfish just came in the other day and the temperature outside here in
Chicago is very cold (15 degrees F). Or is this a bacterial infections. I
also put salt in the water. Main tank has 1 tsp/ 3 gallons. Quarenteen has
1tsp/gal.
How else can I treat this?

Water:
Ammonia ~0
nitrite 0
Nitrate very low less than 10
pH 8.2
Temp 80 degrees