I am sorry you are having difficulty with your naso tangs.
I can tell you from experience that they are very susceptible to parasites. Once established, they can be VERY hardy and long-lived. But many do not make it the first month and almost all of this is attributed to a lack of proper QT/preventative treatment procedures.
I have 14 tangs right now, and I have owned (and killed) many Naso and other tangs over the years by not properly doing what I describe. As soon as you venture away from Zebrasoma tangs (Yellow, Scopus, Black, Purple, Sailfin, Desjardini Sailfin, etc.) you will realize quickly that most tangs CANNOT survive utilizing "ich/disease management" husbandry strategies. What I mean by this, is no QT or treatment for all incoming fish to prevent these illnesses from making it in to your display. There are MANY common fish that can handle ich. I did this for a decade. I would argue that it is not ETHICAL but it certainly can be done with otherwise stellar husbandry--- albeit only a short-term success (1-10 years).
Sometimes, a few hippo can live with ich but many will not. Once you find one with a strong resistance to it, it will likely live a long time. Naso are twice as difficult. Most zebrasoma tangs can handle it, IME. Your yellow tangs probably show no symptoms and have no apparent issues, this is pretty common with these guys. MOST other tangs will struggle. You may succeed, but it will be limited and you'll kill more fish than you will succeed with. You will still kill some zebrasoma tangs because some do not do well with no QT, but IME 2/3 of them can recover and thrive.
My point? With that large of a tank and that many fish, you will want to protect your investment and your livestock. A QT/Hospital tank is a MUST. What you describe sounds to me like velvet, honestly. That said, ich can wreak havoc on naso tangs as well. I've had yellow and purple tangs live with velvet for months over the years, unbeknownst to me. They are very tough once established.
Velvet is running RAMPANT now in the trade, distribution systems are heavily infected it seems and more fish than normal have it before they make it to your door or LFS. I've had two serious run-ins with it in the last 6 months. The first strain did NOT show any standard velvet symptoms - no spots, no dusting on the body, no definitive "signs" I could identify as velvet. I killed angel after angel in my angel tank and some tangs, too and could not figure out the issue. Meanwhile, my wrasses seemed unaffected. I would address one "problem" and was sure each time I did that it was the issue that killed my fish. THEN, after 3 months of struggling with it to various degrees, a Chevron tang FINALLY showed classic velvet symptoms and my jaw dropped. It had been years since I had come across this disease and this one was very sneaky. It killed a naso, hippo, chevron, and several angels that all developed "stress blotches" prior to passing away.
Just a month ago I got it AGAIN. I am 100% sure it was velvet because it reared its' ugly head in a tank experiencing hyposalinity, at 1.007 salinity. It was 100% undeterred and hit and killed FAST. Cupramine took care of the velvet in both cases. My hippo survived in a tank with this velvet (very established I had her for YEARS) for nearly two months before dying. Very sad. This was actually my primary tipping point for switching over to proper QT/Hospital procedures. I have lost 1/100 of the fish to disease since then and I can keep species that I killed often in the past (acanthurus tangs like achilles, powder blue, regal angels, copperband butterflies, moorish idols, and a plethora of others) with unbelievable success.
