Personally I’d love to see your thesis on “stress” manifesting with signs and symptoms of ich/velvet. It’s true that stress can weaken the immune system and allow parasites to proliferate, but “stress” alone mimicking physical, visual disease symptoms…I just don’t buy that. Also, even if your unusual hypothesis is correct there’s no harm in treating for parasites. A fish dying despite treatment isn’t evidence that “stress” was the cause. More likely, the treatment was just too late as is very frequently the case with velvet.
In terms of your last paragraph just because a disease doesn’t run rampant enough to show signs that YOU can see doesn’t mean it’s not there. In fact that’s frequently the case with ich. You can have ich in your tank for years and never know it because the fish are healthy and can fight it to the point you never see it. Until a fish gets stressed or weak and it takes over. That’s the whole idea behind “ich management”….keeping fish healthy enough they can fight it so it’s not a problem, despite it still being there.
I don't have a thesis. Last year I had a velvet outbreak in my 180g after I set it up. My entire fish population was wiped out in less than a week after symptoms started, outside a single sailfin that I managed to save, only because it was too sick to get away. I just saw then how quickly it takes over, and it wasn't just in 1 fish, it was all.
And then I started to QT my fish etc. And during QT I had a naso that got a spot, but it was due to stress after shipping(my best guess), rather than velvet.
I do not know what is wrong with his fish. I agree that the picture looks like a lot like velvet. But what does not make sense is that he says it's been 6-8 months since he added anything to the tank. So how does the velvet enter into the water? If not for that detail, I would assume velvet as well. However, I kept my tank fallow for 2 months as a means to get rid of velvet in the tank, but now 6-8 months and it shows up? That does not make sense to me.
Also another symptom of velvet is the fish do not like to be in the light. When my fish had velvet they mostly hung out under a ledge. I didn't see any mention of being sensitive to light. Also no mentions of flashing, another symptom.
And as for it not running rampant enough - that is certainly NOT my experience with velvet. It does run rampant. It also causes them to have difficulty breathing, which is why I said if the fish shows that symptom, I'd probably pull him, and maybe fresh water dip.
If it is velvet, the other fish will show symptoms soon enough.