My opinion might differ from a lot of the reading I've done as well. I can't say I know what the cause is, but I can easily rule some things out. Carbon being one. I had 7 tangs (I say had, because I'm selling them to get a couple butterflies in, then plan to start over with tangs) in my reef system, none with HLLE but one did have the beginnings of it (on the head) along with fin rot, during a brief stay in QT under cupramine. That was a yellow tang which had already been through cupramine once with no ill effect. However the second time around it was due to a resurgence of ich and I broke down the system to run fallow. That tang unfortunately went straight into a full level (.5+) treatment all at once (2 yrs ago ish). He made it through the treatment and back into my display, and this pic is from only a week ago...
In that system I run marineland carbon which I consider cheap but not bottom of the line. In a reactor, not tumbling. I actually fill the (twolittlefishies) reactor about 3/4 full (not 3/4 cup, 3/4 of the reactor full) on my 260g system and have been changing it monthly. I do rinse it well with ro water. I consider this to be needed 100% especially after my triton results. You can absolutely see the difference in the water if I let it go too long without changing.
GFO, I don't know if people try to put the blame on this really but I'm sure someone has at some point. I also run quite a bit of GFO (phosban) and I would rule that out.
NUTRITION: That would be my best guess as to a cause. My yellow tang mentioned above, was eating everything prior to that 2nd quarantine process. He was really mad at being pulled from my display and moved into a bare (at that time) tank. He stopped eating for a while and only ate nori through the entire qt. This fish ate everything that touched the water, and just stopped all at once, only nori. fins rotted away to almost nothing. HLLE began on the head. I had no intention of ever putting it back in my tank. The day I decided it was time, I caught all the fish it was with to move into the display including him and without even realizing it dumped him back in the tank with the others. Soon as he hit the water I was like oops! but there was no way to get him due to tank size/ aquascape. He immediately returned to eating everything again and fins began growing back. Not too long after he was back to his old self and no signs of HLLE.
I fed nori soaked in selcon which I'm sure helped, but I also fed macro from my sump/fuge which I'm sure helped more. As well as live black worms which I'm positive are the best available food for your reef period. Mysis daily. I feed 3-5 times a day every day. I'm not scared of nutrients. All my tangs are the healthiest around. I feel so bad when I see other tangs because I just see how skinny and obviously mal-nurished they are and people don't seem to get it, more often than not (LFS's but also some hobbiest's stuck on pellet diet). Don't get me wrong I feed NLS pellets as well and I'm sure that they are good, but just lacking what you get from everything else. I still feed nori several times a week if not every day, but I no longer soak it in selcon after I ran out of the first bottle. Did it help? I'm sure it did. Is it needed with all the other items I feed? Probly not, and none of my tangs are going down hill.
Now onto my 125g FOWLR tank... 4 tangs in there. Bad water, never changed any or tested anything. Completely ignored tank for the most part. No carbon, no GFO. No nothing really. I have major overhaul plans for the tank but $ and mostly time are lacking. That said they are all pretty healthy but there is also a yellow tang in there with the beginnings of HLLE. I feed the tank a decent amount but probably half of what I feed my reef. Forget blackworms quite a bit. Could definitely do things better for these fish but I basically want to start it all over again so I don't really even see it much. Thats the cause IMO. I'm sure I could bring this tang back with time, more black worms, and selcon soaked nori. But I have no real interest in the tank and no $$ to do what I want with it so it sits in limbo. I am trying to unload some of the livestock in there to get the ball rolling but it's easier said than done.
Anyway, I just wanted to share my experience with you in hopes it can help you out, or someone else. Also, I took an interest in this thread, because I plan a purple tang when I re-up my tangs. I also plan to buy one with HLLE to start, because I'm not going to pay $240 for what I consider a $40 tang. Thats what I used to sell them for when I worked in a LFS as a teen. I understand why they are so expensive now but I just can't really get past that.
