Alan Myers wrote,
developed some reddish spots at the top of its body
between the dorsal fin and the eyes. It looks like rust! ...

If it isn't mating size and fighting with another male, my first guess
would be a parasitic infection called Costia. The red spots under the
scales are little hemorrhages. QuickCure or any other
formaldehyde/malachite green medication should clear it up.
........................Frank