HI, WELCOME TO FISHLORE!
HITH is thought to be a dietary problem. Specifically a lack of vitiman C. To combat it you need to feed really high quality food, a variety of frozen and flake, and use vita-chem every other day.
Also if the fish have ulcers you will want to treat with maracyn-two, or maracyn plus.....
Food as described above alone, and good water may help, but if the fish have open sores, they will need treatment with medications.
I have heard that using carbon in filters on a regular basis also contributes to this because it pulls too much of the minerals out of the water....
I never run carbon in my filters unless I'm removing medications. Then after 24 hours I remove the carbon and go back to the bio-filtration of floss, sponges and bio-balls....