1) It is not a theory that Acanthurus tangs particularly powder blues, Browns, Achilles, and goldrim have significantly less slime coat to protect them than other tangs.
2) I have experience with ich management (mostly successful) over the last 12 years. Horrendous success with Acanthurus tangs, however. See my thread below, actually read it.
3) your logic that the fish can fight the parasite in an aquarium the same as they would in the ocean is flawed at best, with all due respect. In the ocean Acanthurus tangs swim miles and miles each day which means the parasites have a much more difficult time getting to them,
But MOST importantly, being enclosed in a small cage where the parasites can reproduce exponentially faster, is a vast difference. In the ocean, the number of parasites on the fish would not increase exponentially as they do in a small glass cage. The fish can build a resistance to it as the natural population increases very slowly and has far more difficulty reaching hosts.
4) to assume that even the most seasoned hobbyists provide an environment of low stress to this genus of tang (given their swimming needs), or really any fish for that matter (relative to the ocean) is preposterous.
I love this hobby but nothing I can do will be as stable or ideal as their natural environment that they evolved to inhabit. Period.
Here is the thread outlining my experience, that's all that it is. It's pretty astonishing though and you should read it. It will tell you that I didn't qt anything for a decade and relatively successfully managed ich except with regard to these fish.
https://www.reef2reef.com/threads/i...ears-of-experience-and-ich-management.206347/
Tangs that are pelagic and/or schooling would be two of the kinds I was referring to that almost noboby can provide suitable habitat for, in a nutshell because we know so little about them...just enough to know how unsuitable they are for tank life, in fact. A chain of six foot tanks wouldn't do it. Your group of tangs falls into that category. Keeping one of these in a tank is a stunt IMO.
On #1
A thin slime coat just means you need to be even more conservative in dealing with these species...generally not selecting them because virtually any tank would be too small for their lifestyle. "More than theory" implies this has been demonstrated scientifically vs just hearsay. Remember that thin slime coats come from stress and malnutrition...so I'd be curious to see any sources explaining this phenomenon in this genus.
On #2
I did read your post - very common experience and I am glad you shared!
On #3
Exposing a weak fish to outbreak levels of ich implies that several bad things have already happened (See #1 again.), which would sap the slime coat of any similarly treated fish.
Think about this:
If a wild pelagic tang like we are talking about travels a mile in a given day and you put him in a six foot box to live, that environment is about 1/1000th the size. But it's not all that uncommon to see it done.
To put a clownfish from the wild that lives in a single square foot through the same ordeal you'd have to squeeze him into a .3mm box to live. Why is that only obviously ridiculous when we talk about the clownfish?
On #4
If I didn't say that about this genus too, I meant to.
Being conservative (and honest with ourselves) when picking fish would pretty much exclude this whole genus...along with the many other genuseses of marine wish we just don't mess with.
Extrapolating this thinking to all oceanic fish is fallacious as there are obviously many whose lifestyles are not at all pelagic, nor even schoolers, and which do conceivably (if not perfectly) fit inside a typical aquarium. Even some other tangs.