Very common in Oscars, although other types of fishes can certainly be infected, including Discus.

• Hexamita once reported to be "the cause" of this syndrome.
• Hexamita has been found to be an occasional contributor

Other contributors include:

• Crowding or more accurately: Too small tanks
• Too infrequent water changes
• Sludged/fouled gravel beds
• Overgrowth of bacteria [UV requisite]
• Refined diet and deficiency
• Genetics

Diagnosis: Erosions and pitting of the head, usually around the nostrils at first but can erode the whole head and face before killing the fish. The lesions are usually of a pale gray color and not red.
Progression: Slow
Prognosis: 75% plus cure rate can be expected. Not every fish heals and appears normal. This would depend upon the severity of the lesions as conditions are improved and treatments are instituted.

Treatment:

• Ensure perfect water quality with a regimen of regular water testing for pH and Ammonia. Ensure a low Nitrate level by testing, and application of plants or allowing algae to proliferate on tank furnishings and glass.
• It is easier to maintain fresh healthy water for your larger fish which may be suffering with Hole in the Head by placing them in larger facilities. No Oscar should be grown to maturity in a 55 gallon tank ro smaller, while it happens all the time. Oscars do much better in a 75 gallon tank or larger. If un-crowded, you will find Hole in the Head a rare disorder among Oscars indeed.
• Perform regular water changes to make sure water remains fresh smelling and healthy. 20-30% changed per week is a great ideal, but folks who have daytime jobs sometimes do as well with 50% every 14 days.
• Make sure that if there is gravel or other substrate in the tank, you should be sure it remains regularly siphon-cleaned. Use of a Python water changer allows you to maintain healthy substrate by facilitating deep siphon cleaning.
• Inexpensive UV [ultra violet] sterilizers are an advantage in the maintenance of clear, clean water by decreasing the growth of bacteria in the water. These bacteria are not inherently pathogenic to fish, but can stress them nonetheless.
• Feed a varied diet. If your affected fish enjoys live foods, feed SuperWorms [PetsMart/Armstrong Cricket Co. $2.99 per twenty five worms], MealWorms, Earthworms, Brine Shrimp, Blood Worms and other frozen delicacies. An occasional small feeder fish served "live" is good. Pelleted diets can round out a diet variety. Pellets are very nutritious but are no substitute for occasional live or frozen fare.

Chemotherapy

• Finally, treatment with Metronidazole can help in the clearance of these lesions. Some have maintained that the Metronidazole clears the putative Hexamita. Still others argue [probably correctly] that even in the absence of Hexamita, the Metronidazole will control bacterial invaders of several types and assist wound healing. Finally, the consideration of "what harm?" in which Metronidazole is well tolerated by fish in the water and in the food. We have used Metronidazole implanted in [hidden in the bellies of] the fish we fed to Oscars. We have also used it in the water at a rate of 500mg per ten gallons every day after a twenty five percent water change.
  When hiding Metronidazole in the bodies of the feeder fish, use a dose like this: Sixty seven milligrams of Metronidazole per six inches of fish being treated once per day. So a foot-long Oscar would get a half tablet of 250mg/tablet strength Metronidazole in it's daily feeder fishy. Common sense would dictate the feeder fish would be deceased when the tablet is implanted. The Metronidazole is given daily for five days.