Large Red sore on Koi

I have a large blue koi that has a big red sore near his tail, there is another
small
one under his eye. I have been using Medicarp medicated food and Melafix liquid
that I've put in so far for 5 days, they say do it for 7. You can also use it
longer if you have to.
The fish seems fine, it is eating and swimming around, anyone ever have
anything like this?
Any feedback would be appreciated.
Carole
NJ