Strangely sick fish???

My ryukin Dorothy, always the healthiest and most active of my fish, is acting
very sick. This has been going on for a week or so. Water levels are all
fine, I have two gf in a 20gal tank with a Penguin 170 filter. No major
changes in tank recently. I do a weekly 30% change, and at the weekly change
my ammonia and nitrites are at 0ppm, and nitrates are usually around 20ppm. My
other gf, an oranda, is acting normally.

Symptoms: the fish swims on its side, curled up with head and tail nearly
touching the bottom of the tank. Swims in a circular pattern. Only swims if I
turn off the filter (which I have been doing in order to get the fish to eat).
Otherwise the fish sits upright in one corner. I suspected a swim bladder
problem, but the fish sometimes swims normally and and never floats at the top.
If the fish gets in the air bubbles, it flips over and floats up to the top
before escaping the current. I noticed this morning that one of the fish's
eyes is normal, and the other seems pressed in. Both eyes look fine otherwise,
and the fish seems to be seeing with both eyes.

Any ideas? I've had this fish just over a year, and she is probably about 3"
long now (was about 1.5" when we bought her). I really don't want to lose her.