Evening all,
Implying unless you medicate them you will not be able to keep them alive is a bit of an overstatement. We will always need treatment plans for things. Taking the approach a fish has no immune system of their own to get better is a bit shortsighted (if I used that word correctly:)) and only meds can cure them.
As for a repeatable way to keep fish alive, it is thru food and environment. And (wait for it) ....don’t qt. None of mine get Qt. Last addition was my file fish I dumped in last month, already gone thru 1/2 my aptasia.
I had 3 tangs go thru either bad ICH or maybe brook/velvet and now are fine. It took 2 to 3 weeks as it went from one to the other, but it is now gone. My Vlamingi also decided it would be fun to have popeye. It started last week, for most part gone. The commonality is no treatment. I believe by pulling them out. Dumping heavy meds that stress them out reduces their strength causes so bad as much as good.
This was when it first appeared, it got worse before gone.