I googled stress-induced hyperthermia and got very little.

That would imply that if it's a marker for consciousness it's not a well accepted one. Maybe it's one marker among many, but certainly not enough to support the HN title "Fish have feeling".

And simply on a logical level I'm not seeing the connection between stress causing heat, and consciousness.

Animals obviously can get stressed, and animals obviously can have a fever. I'm not seeing why consciousness is necessary to link the two.