I've also heard that carbon can cause HITH disease...But both my Oscars that I've kept since small juveniles have been fine with the carbon...Down the road I might just choose to use ammonia remover instead of carbon in my AC.
I really just think HITH is caused by bad nutrition and poor water quality.
if you have a small tank,its good to use carbon and replace is periodically,obvoisly a big tank wont be affected by small amount of expired carbon,put huge amount of carbon there and im sure that it will affect the water quality