Just out of curiosity how long did you run the carbon for? From what I understand about HLLE is that it's progressive, meaning it doesn't just happen overnight so to say. I may be wrong though..
I have a 90 gallon display with a 70 gallon basement sump and 20L refugium.
I ran a filter sock on the overflow return to the sump. It had about 3 cups of activated carbon in there.
Noticed both tangs, blue and yellow acting funny within a day. Clowns started to act funny as well. This was after a large water change as well.
Tangs showed signs of HLLD 2nd day and progressively got worse.
Figured it was the released"toxins" from the flat worms.
Blue Tang jumped one night.

Removed carbon.
Clowns improved with in 1/2 day.
Yellow tang took a week or more to recover. Now back to looking great again.
I've been at this a reefing stuff a while. Seen many instances of the same thing happen after using carbon. Maybe I used too much, don't know.
Only know that next time,if any, water changes will do.
Just my 2 cents.