*disclaimer* I don't know anything about any of your fish specifically and I'm not a chemist...
That being said, do you have a test kit (ammonia, nitrItes, nitrAtes, ph, etc...), if you do, check the parameters and let us know.
That quick of a rise in temperature (assuming your normal temp is around 76 or so?) could most definately cause stress, but I wonder if it would also mess with the ph in the tank? Most fish can get sick if there is a huge, quick ph swing.
If his scales are red almost bleeing looking, all over his body (rather than in just one area), then I would be willing to be it's not caused by a tankmate bothering him, but rather from something in the water or maybe some kind of parasite or fungus.
I'd say first thing to do would be to do a nice water change (if you have a test kit, test the water before changing it, and then again tomorrow at the same time of day.)
He's eating, so that's a pretty good sign.
sorry, I'm not too much help... hopefully someone else can let you know what exactly it is ...
~Tara