well...how abt yr other fish ? If they are still fine then I think it's nothing to do with salty or temp,however I'd be better to check yr water quality again.In my humble opinion if yr water quality is still good(especially no2 level),perhaps yr Yellow Angle is infected. And I always put an infected fish in fresh water with Tetracycline for 0.5-3 minutes( one time every 2-3 days), it always works for me.
I also use this method to get rid of parasite but don't have to put Tetracycline in.
may anybody comment my method ,if you please...