There are two forms of HITH.
First is caused by a protozoan called hexamita or spironucleus. This is the less common reason for HITH.
Second is caused by the environment that the fish is in. Water quality, diet, stress or even high levels of copper in the water can cause it.
Both look the same, although the HITH caused by the fish's environment is all external symptoms.
If it is caused by the protozoan, then it is internal as well. The external symtoms are the side effect to the protozoan.
To determine which one it is, you go to the fish's feces. In a fish with HITH, caused by hexamita or spironucleus, the feces will be white and slimy, and the fishs appetite will decrease or it will stop eating all together.
In a fish with HITH that is caused by environment, the feces will be normal, and eating paterns do not change.
Treating for the protozoan when this is not the cause will just make it worse.
Sounds like yours is caused by environment. Water changes, and a change in diet should do it.