Here is a page from my Log book in 1976. Five years after I started my tank. These fish were all in my "40 gallon" tank.
Some of therm seem to be ich magnets to me, and yet none of them died from ich or velvet.
And that was over 40 years ago.
This was in the early stages of me learning what to feed and way before reef tanks, when we only had dead coral skeletons for decoration that we removed and bleached every two weeks. I am surprised anything lived.
We even used to try to cultivate aiptasia because it was the only invert around, and you had to buy them as "Anemone rocks".
When I talk about healthy fish or keeping them healthy I am not speaking of last year or five years ago.
Five years isn't even half the age of a hermit crab.
I have been doing this since way before last Tuesday and if your fish are immune, I mean they are immune "forever". No matter what fish you keep of what kind of tang.
They did die from things associated with my stupidity. My Hippo tangs then did get HLLE which I assume I would get also if someone put me in a 40 gallon tank for 10 years with moorish Idols, puffers and French Angelfish.

I killed that French Angel after 4 years by accident. So he lived that long in a tank with a copperband that seemed to have ich or velvet.
It looks like this copperband Had some sort of parasites when I bought him as he was scratching.
I don't remember what I did to cure him or if I even did anything.
