The colors have faded a little bit over night. Still breathing. No change in swimming.O.K., this one is a puzzler. I re-read your message and came across this: " Yesterday he was doing great, eating, swimming. Today he won’t eat and doesn’t look good." Normally, when I read this sort of thing, I just assume that the person missed the early clues of a problem and it just APPEARED to turn up overnight. In looking at your video, I'm now thinking that this fish looks pretty good overall. Flukes, ich and other external parasite will cause a more gradual worsening of its condition. In addition, as I said, other fish are normally affected as well.
It is rare, but the fish may have suffered an impact injury - gotten frightened and ran into the tank wall. I'm trying to think of what other issue could cause symptoms overnight like that.....there really isn't anything else....
How does it look this morning?
Jay