Well, "Mr. Fishy" went back to his owner on Saturday.
Today I see the owner and I'm told the fish hasn't eaten.
The fish ate Saturday night, but not since then.
I ask all sorts of questions (learned from this forum!) and it appears the owner did everything right, though the temp of the water isn't known. When I had the fish, I kept a small intense lamp over the water for several hours in the morning and evening and that helped warm the water and the fish was very active.
I suggested the owner join this forum and ask for help or to please give the fish back to me. I don't know if "Mr. Fishy" is still alive as I write this, but I sure hope so.