Success with Aqua MedZyme
First off, I know almost nothing on treating Koi diseases, etc and my only other attempt at treating a sick fish (last year) resulted in a dead fish.
That said, this year I had another Koi that developed a couple of fuzzy whitish patches and lost some scales. I found recommendations for Aqua MedZyme natural bacteria treatment on the internet and used it to treat the entire pond (I have no association with Aqua Med products). All fish recovered to full health. Great! Much easier to treat the entire pond rather than try and catch the bugger and put him in a quarantine tank. That process last year seemed to result in more trauma to the fish than the disease did. He did not want to be caught.
The package recommends preventative treatment in the spring and fall and when a new Koi is added to the pond, and based on the ease of use and my recent success, I think that I am going to add it to my regular preventative maintenance routine.
If anyone else sees a reason that this should NOT be used as a preventative treatment, please comment below.