Well my experience with carbon was the tanks i ran with carbon years ago the fish seemed to get hith, and the tanks i didnt never got it. A lot being the same species. All the tanks were on the same maintenance schedule, same water, all CA/SA species. I never use buffers.
At the time i had a really big batch of tapajos redhead juvies. Half the batch got hith, and split tail. I ran a lot of carbon on that tank. Uncle Ned from Uncle neds fish factory came over, and pointed it out, and explained there was a lot of discussion going on about carbon dust being the culprit. Since that day years ago i havnt ran carbon on any tank period. Never had a case of hith again.
I know a study proved carbon was the case in a saltwater fish study, but there's argument against that study for freshwater fish, cause they were salt water.
After that article i'm almost convinced its at least one highly suspicious culprit, but i do agree with some other arguments too. It's such a mystery still which i find odd.
I also read carbon removes vitamins, and trace irons, and other minerals from the tank water too which could be a combo factor as well :-??