HITH is caused by stress, and in a glass box stress can come in many forms, for many different species of fish. In the case of blackwater species I agree with both Matt, and Tom, in that those species found in acidic blackwater environments do not always cope well with various parasites/pathogens found in non blackwater environments. Why do some fish get HITH while others kept in the same tank, and fed the exact same diet do not? As previously stated there is much more than just pH involved, and even tannins, various other natural turpines and chemicals found in indigenous plant matter may not only keep the pH buffered quite low, but in some instances (such as Terminalia catappa aka Almond leaves) some of these various indigenous plant matter are also known to produce antimicrobial activity that suppresses both gram positive & gram negative bacteria. Take those species out of that "protective" environment, and introduce them to paramaters outside their ideal range, and a fish that comes under stress (of any form) is going to be open for invasion from any number of pathogens that it would not normally encounter in the wild.
