I honestly think if the parameters are always good and it is being fed what it needs to stay healthy, it is a stress issue. That Tang being in that size tank just doesn't work well. Tangs need lots of room to cruise around. Almost every case of HLLE
that you read about has a Tang in a smaller than 6' tank or the person wasn't getting greens in it's diet. IMO
, that is not coincidence. I would suggest rehoming it. Probably not what you want to hear, but the fish would be better off.
The carbon idea comes from a research project that had everything constant except one tank had filtration with carbon and those fish got HLLE
. The fish in the tank without carbon did not. I'm not sure that explains it all, as other research has found malnutrition and/or stress to cause it too, but it is interesting.