Carbon has been implicated in HITH, though not directly proven, removing any carbon from your filtration is a good idea.
HITH is mostly caused by water quality, secondary is usually diet. If the tank has been poorly maintained don't start with large water changes yet. 10% daily for a week, after that 25% to 50% for a week.
Vitamin deficiency is also a big contributor. If you are feeding live feeder fish, stop. They have very little nutritional value. Get a quality cichlid pellet, and a liquid mutivitamin, usually found in the infant section of most drug stores. Put one feeding worth of pellets in a small glass, shotglasses are good for this. Put several drops of the vitamins on the pellets, and let it soak in for a good 15 minutes. Feed this once daily, nothing else.
Internal protizoans have not been directly linked to causing HITH. These are often found in cichlids, in the normal digestive floura. Healthy fish get by just fine with them. They will multiply faster than usual in a stressed or ill fish, and are often the cause of cichlids going on a hunger strike. They are found in the digestive tract in excessive numbers in fish with severe HITH, only because the fish is stressed from HITH. Internal protizoans have nothing to do with external manifestations on fish such as HITH.