Sad news to report. The golden, double-comb betta passed away.
I change his water frequently and there were no nitrates or nitrites. I have also been checking the temperature, and it is well within the correct range.
I added Betta-Fix to his water, but that didn't help at all.
Then I found him floating along the surface this morning when I went into the kitchen. His tank was on the counter between the kitchen and breakfast nook, where he could keep an eye on everything.
He may simply have lived out his little life.
May he rest in peace.