I have notice blister like marks on the body of some of my bigger Koi. It looks like blisters in the mucus layer and seems to move around, I am also sure one will be able to rub this off. I have been told of keepers that have added 6% salt to the pond and it disappears. I believe that Sakai refers to this as a koi cold.
Anyone with more experience on this and an a alternative treatment as I would prefer not to add 200kgs of salt to my pond.
I have also experienced more Mucus on the head than normal.