Hi Kathy, I would keep the fish isolated, at constant temp with plenty of airstones, although with a koi of that size ammonia will be an issue in a non-mature tank. Can you upload any pics. It does sound like a bacterial inf. again being a large fish and prolonged onset of inf, I would try an bntibiotic. Baytril is the usual type, other types mixed with food. Only vets will provide this. Plus the vet should give you some sterile swabs, take a swab from the gills and from the sores, send to a disease lab to confirm. They should run an antibiogram in culture to determin the best AB, possibly follow up injection. It does sound as though it is advanced. The skin scraping after treatment wont show much, It doesnt sound viral, but then again shouldnt be ruled out. SVC occurs in the spring.