If there is no visable parasites on the fish i would go for gill flukes, the finrot is a secondary infection which is common wthn parasites, hole in the head starts off as pitting, like a pencil dint in the head.
Thanks Wilder. The HITH symptoms on this one koi, has been obvious and has not deteriorated in the last 2 years The water peramaters cannot be faulted in either pond. I think it has to be gill parasite problem as you say. Saying this, it is easier for my dad to catch and priest trout than it is for him to catch and inspect his own fish. I dont think I could do it. Unless they inspect them then I suppose there is little else I can do? I just hate to see them suffer; they have been living for nearly as long as Me!!!!
Do a partial water change on the pond or tank...Stay with the salt treatment...any non iodized salt will work... Id give them a stong dose of furazone green and leave em sit for a few days...
Change the water at 50% if any signs of stress.....