(This may not be new info, but I thought at least the links would be intersting or useful. I cross posted this to a user-question thread in the regular fish forum earlier.)
Would too many people explode if I said that your fish can gain
immunity to pathogens by being infected with
non-related non-pathogenic organisms? :D
https://scholar.google.com/scholar?hl=en&q=Tetrahymena+pyriformis+immunity
And how many non-pathogenic microbes are in seawater?
Apparently just the right amount!
https://www.garlandscience.com/res/pdf/9780815365174_ch01.pdf
That's a lot of microbes and a lot of pathogens.....and the fish apparently need them!
Repeat after me: Immunity. Immunity. Immunity. Immunity. :)
Contrarian Sidenote: One of my more interesting customers at the LFS where I used to work, when he would buy a fish or coral, would ask for an extra large bag with lots of extra water from our system just so he could seed his tank more. Back then, even I looked at him funny and questioned him every time he did it....makes a lot more sense now!!