In the Discus world on Simplydiscus we do not use carbon for the specific reason we believe that it contributes to HITH. We use it to
take out things in the water such as medications and so forth.
It is believed that carbon does more harm than good as it takes out important nutrition in the water.
HITH is a stress induced disease caused by an intestinal flagellate protozoan, usually Hexamita, and not a bacteria. Secondary bacterial infections can occur in unsanitary conditions. It is related to poor diet, especially lacking vitamin C, and poor water conditions. Many fish have latent infections of the prtozoan, and when stressed, the populaion of bugs explodes. Healthy fish that are not otherwise stressed, manage finely with a few bugs in their gut.
Constant reccurances of HITH should (or at least ought) to point out to the fish keeper that there is an enviromental problem in their tank, be it overcrowding, temp, pH, conductivity, bioload and/or a nutritional problem.
I have raised many Discus
without carbon in my tanks. If I do use it, I never use it for more than 24 hours.