HitH is usually caused by a lack of vitamin K. ( Untergasser) This is caused by poor diet and or an internal parasite ( hex) and or bad water.
Melafix is a waste of time & money imo, no point in just treating the symptoms , not that melafix will do much for HitH anyway, tho it may suppress secondary infections of the head craters, you need to treat the cause.
hexamita ( an tiny gut nematode) is treated with metronidazole, sold at your lfs as Hex-a-mit. You can also add it to their food if they are eating.
Carbon should only be used temporarily to remove meds at the end of a treatment. it gets saturated and can cause problems and as stated earlier, can also remove valuable trace minerals, including vit K. Different carbon types from different wood will remove different things but all will remove meds.
Use a hospital tank. change at least 25% of the water before each dose, use daily instead of every other day.
Hex is very common in cichlids, it is also killed at temps above 90F for 3 days.
Metro is a human antibiotic and works by breaking down the cell walls of the nematode
A combo of heat and metro should clear it up....or not?
If you do increase the heat the fish will be OK if you also greatly increase the airation, as warmer water holds less oxygen
hth