I gave you the best way of attacking fungus via @Andy41 case a while back Wayne the outcome was a successful one Andy's problem was caused by switching off his filtration at night so as not to upset his neighbours [thats if my memory serves me right], you said you brought your koi indoors Question did you have trouble netting the koi in question?I brought them inside the garage for the winter.
Also when transfering them across did you exactly match temperatures of the water, plu are the indoor filters mature enough ?
There are lots of ways this infection could have come about yo'll have to go through everything you did to come up with the answer .
Then dont make it again
Dave