I did a big water change and gravel vacuum, the third in five days. He’s listing on the bottom of the tank today; I’m not optimistic. My money is on hexamitiasis/‘hole in head’ disease. I had a fish in the same tank with it a few months ago. General cure worked well on her, but I am guessing it may have lingered in the tank at low levels, and the stress of the high temps weakened his immune system enough that he got sick.
This guppy is the last of our store bought ones; the rest are all second generation. Hopefully they will be more resilient to our rookie fish owner errors.