Thank you guys. You're all wonderful.
It came as a shock because even though he wasn't improving as fast as I would've liked, he seemed normal. He was eating and swimming around and, after the last update, he had perked up considerably and was starting to act like his old lively self again. Water parameters checked out fine, temp's was good. I actually spotted some fin regrowth this morning. This would have been day six of AQ salt treatment to catch fin rot early in case he had it.
I hope this isn't too morbid to put up, but I always loved his eyes. They had so much life in them. Even now, they still do.