I would try to vary their diet. I've had many tangs and none of them ever had HLLE. Maybe I was just lucky? I adopted a Sohol and purple that looked like someone took an Ice pick to their head for a cruel joke. I fed all my tangs nori, formula two pellets, krill, silver sides and mysis. After about 6 months you couldn't find a mark on them if you tried. I always had a few med. sized rocks of hair algae growing in the sump to keep feeding/grazing as natural as possible for them. When it got to be about 2" long and covered all over, I would put it in the DT and they would go crazy for it.