Hexamita / Hole-in-the-Head
Symptoms
Pitted lesions on the head, particularly above the eyes and along the lateral line — start as small pinpricks and erode into deeper craters that may leak pale stringy mucus. White, stringy feces. Loss of appetite and weight loss despite the fish appearing to feed. Color fading. Most pronounced in Discus, Oscars, Severums, large cichlids, marine angels, and tangs.
Causes
Flagellate protozoan that lives in the intestine of many healthy fish but only becomes pathogenic when the immune system is stressed. Triggers: chronic poor water quality (especially high nitrates >40 ppm), nutritional deficiencies (lack of vitamins C and D, monotonous dry-food-only diet), and overcrowding. The 'hole-in-the-head' lesions are bacterial secondary infections following Hexamita-induced erosion.
Treatment
Metronidazole is the standard treatment — added directly to the water for 5-7 days at the label dose, OR mixed into food (more effective for the intestinal phase) at roughly 1% by weight of food. Heavy water changes throughout. Simultaneously fix the underlying causes — drop nitrates below 20 ppm, vary the diet with frozen and fresh foods, reduce stocking. Severe cratering may benefit from a topical antibiotic.
Prevention
Keep nitrate under 20 ppm — single most important factor for Discus, Oscars, marine angels. Feed a varied diet including vitamin-rich frozen foods. Avoid overcrowding. Quarantine new arrivals.
Notes
Cichlids and large reef fish can recover dramatically — the cratering scars over and refills in months once Hexamita is cleared and husbandry improves. Don't give up at first sight of lesions.