Everybody has an opinion, this time thru my experience I am siding with X.
I have experience with infectious disease in humans. Remember the "boy in a bubble?" His immune system failed opening him up to infections in the environment that we would never notice or succumb to. He had to live in a perfectly filtered artificial environment. We stress fish sometimes, their immune system (slime coat) fails and it can put them in the same position, there can be no infectious organisms of any kind in the tank. We use UV
light and go thru lots of exercises to try to eliminate every one of these organisms. Our tank becomes the boys bubble. It is far better to bolster a good immune system than to try to live in a sterile environment.
If you want disease resistant fish, it starts with reducing their stress.