Thank you so much for the replies, I truly appreciate it!
He looked perfect yesterday. This one probably got stuck because he's had a swim bladder problem since I got him, maybe 2-3 years ago. It got a bit better when I started feeding him Hikari's Lion Head food- I was told the sinking food would help it, and it did. But he never fully recovered.
Do you have a liquid test kit? Yes
Have you tested your water parameters for ammonia, nitrite, and nitrate? 0 ammonia, 0 nitrates, 12 nitrates (nitrates have gotten to 25 in the past, I always test 0 for the others. It was cycled before I put them in) 6.8 ph
How big is the tank? 40 gallons
What other fish do you keep? 2 more ryokin, 4 cories, 1 bristle nosed pleco. A bit over crowded, I know, but I tend to have tanks that are a bit like that- and fish that live a long time. I just do a little more work to keep the conditions good.
This came on so suddenly like that? No signs of shredded/torn fins yesterday? Any signs of other fish attacking/bullying this one? Yes, he looked beautiful yesterday. No blood streaks, great fins, good color. He was fine when I turned the lights off. The fish have been together a long time and never bully each other. The big one, that I bought at the same time as this hurt one, actually kept going over and sitting on the bottom by him after I freed him. It was really cute, the big one held very still.
I moved the hurt one into a hospital tank though, because when the big one would sit by him, he would eventually have to move, and his tail would brush the hurt one off balance. (they're not terribly graceful) The hospice tank is 10 gallons, I used 7 gallons of water from his big tank, a overhanging filter from another tank, I have a heater in there (what temp should it be at? I have it at 72, his big tank was at 70 without a heater) a small amount of gravel- just so he has something to prop himself up with, a bubble wand, and a tsp of aquarium salt.
I want it to have good oxygen levels, but I'm afraid of the current being too strong because he really only seems to be able to prop himself up on the bottom. Any ideas besides the air stone?
About food and medication: I have some Medi-Gold koi and goldfish food from back when I first got them. His now big friend got really ill right after I brought them home, so I got this stuff. It's got good ingredients and vitamins plus sulfadimethoxine and ormetorprim sulfa. I don't know if it might be a bit too old though.. 2-3 years? It still looks fine ...humm...