This is an older thread and I hope the fish are healed by now. We had a koi pond with bullheads in it. Red sores are usually bacterial infections and not fungal. But fungus can attach later. My guess is that the bullheads who like to stay on the bottom of the pond are exposed to more pond debris. We put in a bottom drain attached to a filter pond to keep the pond floor clean and keep the water circulated. We also did not keep rocks in the pond because it was too hard to keep clean when the debris collected under it. For the new house my hubby will not let me put in another pond because he had to fill the old one in when we sold our house. Nobody would buy it wilh a large Koi pond in the backyard. :(

take a picture of the red sores and bring to a reptuble pond/fish store.

They should be able to tell you what's going on.