Not all activated carbon is the same. The cheap stuff made from coconut and palm trees is the worst. High quality pellets are far less of concern, but even those should be excessively rinsed and placed in such a way in a filter that no movement and abrasion of the pellets can occur.
Another cause of HLLE is malnourishment. All tangs need plenty of algae, ideally fresh. Red algae are preferred. Unseasoned sushi nori is a good substitute for red algae.