Thanks for checking in, Leilei. Unfortunately, I think he may have passed away (I say "may have" because I once declared a fish dead that actually turned out to be alive). In this case, he has been lying in the same spot on the bottom of the tank since late yesterday afternoon and I can't detect any gill movement at all. We've got a busy day ahead and I was planning to do the water change later this evening, so I think I will stick with that plan and by then either my suspicions will have proven correct :( or not.
This is very sad for me because he was so healthy until 2 days ago and I have absolutely no idea what might have happened. I am getting suspicious that it's something about the kitchen because over the past year, 3 different fish who have lived in our kitchen have basically died from what seems like stress (and they were all living in different tanks, so it's not something to do with the habitat). One was actually fine as recently as 15 min before I found him dead. Maybe I need to find a different location for our next fishy....