Parasitic Diseases 5 min read

Hexamita / Hole-in-the-Head

By Editorial Team · ·Reviewed by Dr. Elena Rossi

A parasitic infection causing facial pitting and lateral line erosion in cichlids, driven by poor water quality and nutritional deficiency.

Close-up of discus with Hole-in-the-Head pitting lesions along the lateral line

Quick Answer

Hexamita (Spironucleus spp.) is a flagellated protozoan parasite that infects the intestinal tract and lateral line sensory organs of cichlids and discus, producing the characteristic pitting lesions known as Hole-in-the-Head disease. This is a medium-to-high severity condition because early lesions are reversible with treatment, but advanced erosion damages bone and leaves permanent scarring. Start metronidazole at 250 mg per 10 gallons every 24 hours for 10–14 days, delivered via medicated food for maximum gut concentration, and reduce nitrates below 10 ppm through aggressive water changes. Every week of delay allows the parasite to erode deeper into the lateral line canal, and secondary bacterial infections such as Columnaris often colonize the open pits.

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Treatment Options Compared

MethodEffectivenessCostTime to ResolveFish/Pond Stress Level
Metronidazole in medicated foodHigh – delivers therapeutic gut concentrationModerate ($15–25)10–14 daysLow – minimal tank disruption
Metronidazole water-borne treatmentModerate – poorly absorbed through gillsModerate ($15–25)14–21 daysLow – requires daily dosing
MetroPlex + Focus (Seachem) in foodHigh – binds medication to food for gut deliveryModerate ($20–30)10–14 daysLow – gentle on fish and biofilter
Water changes + vitamin supplementationLow–Moderate – addresses environmental triggers onlyVery Low ($5–10)3–6 weeksVery Low – essential baseline care
Nitrofurazone + Metronidazole combinationModerate – covers secondary bacterial infectionModerate ($18–28)10–14 daysLow–Moderate – dual medication load
Troubleshooting Guide
1 Pits stopped growing but are not healing after two weeks of metronidazole.

The parasite is likely cleared, but tissue regeneration requires pristine water and nutrition. Increase water changes to 50% twice weekly, add vitamin-soaked foods, and ensure nitrates stay below 10 ppm. Bone-level erosion will not reverse.

2 Fish refuses medicated food but shows no improvement with water-borne metronidazole.

Water-borne metronidazole is poorly absorbed through gills. Force-feeding or soaking regular food in a concentrated metronidazole solution is more effective. Mix 250 mg into 1 tablespoon of water, soak pellets for 30 minutes, and feed exclusively for 14 days.

3 Stringy white feces cleared but head pitting continues to worsen.

The intestinal infection may be resolved while the lateral line infection persists. Continue metronidazole for an additional 7 days and add a broad-spectrum antibiotic such as kanamycin to prevent secondary bacterial colonization of the open pits.

4 Tank mates show no symptoms but the discus with HITH is deteriorating.

Hexamita affects stressed individuals first. Test water parameters immediately and correct any nitrate elevation above 10 ppm. Isolate the affected fish in a hospital tank with daily water changes to reduce parasite load and allow focused medicated feeding.

Glossary of Terms
Hexamita
A genus of flagellated protozoan parasites that infect the intestinal tract and lateral line organs of cichlids and discus, causing malabsorption and head pitting.
Spironucleus
A related genus of diplomonad flagellates often implicated in Hole-in-the-Head disease alongside Hexamita; both genera cause similar intestinal and lateral line pathology.
Lateral line
A sensory organ system running along the side of fish that detects water movement and pressure; the open sensory pits in cichlids are a primary site of Hexamita invasion.
Metronidazole
An antiprotozoal and antibacterial medication effective against anaerobic flagellates such as Hexamita; best delivered via medicated food to reach intestinal concentrations.
Flagellate
A group of protozoans possessing whip-like appendages for locomotion; Hexamita and Spironucleus are flagellated parasites that colonize fish intestines and sensory organs.
Scientific References
  1. Hexamita and Spironucleus Infections in Ornamental Cichlids
  2. Hole-in-the-Head Disease: Etiology and Treatment in Discus and Oscars
  3. Metronidazole Protocols for Flagellated Protozoan Infections in Aquarium Fish
  4. Nutritional Deficiencies and Lateral Line Erosion in Freshwater Fish

Frequently Asked Questions

Is Hexamita always the cause of hole-in-the-head?
Hexamita (Spironucleus) is a primary contributor, but environmental factors like poor water quality and nutritional deficiencies are equally important. The disease is multi-factorial.
Can HITH be reversed?
In early stages, yes. Pits can completely fill in over time with clean water, proper nutrition, and treatment. Advanced erosion may leave permanent indentations.
Why are discus so susceptible?
Discus have a delicate slime coat and are extremely sensitive to water quality. They require very low nitrates and a varied diet. Any shortfall can trigger Hexamita proliferation.
Does activated carbon cause HITH?
The theory is that fine carbon dust irritates the lateral line and sensory pits, opening them to infection. While debated, many experienced keepers avoid carbon in cichlid tanks or ensure it is thoroughly rinsed.
How long does MetroPlex treatment last?
Typically 10–14 days when fed in food. Continue for at least 5 days after symptoms disappear. Water-borne treatment alone is less effective.
What vitamins help prevent HITH?
Vitamin C, vitamin B complex, and trace minerals like zinc support skin and immune health. Soak food in a liquid vitamin supplement like Boyd Vita-Chem or Seachem Nourish.
Can HITH be prevented with water changes alone?
Clean water is the most important factor, but diet is also critical. You need both low nitrates and a nutritious, varied diet to fully prevent HITH in sensitive species.
Is Hexamita contagious?
Yes, the flagellate can spread through the water, especially in crowded tanks. However, it usually only causes disease in stressed fish with weakened immune systems.
What foods are best for recovering fish?
High-quality frozen bloodworms, brine shrimp, and mysis shrimp soaked in vitamins. Medicated pellets containing metronidazole are ideal. Avoid dry, low-nutrient foods.
How does Hexamita affect the fish's digestive system?
It infects the intestinal lining, causing malabsorption. This leads to the classic stringy white faeces and a sunken belly even when the fish eats. Treatment directly targets the gut.
Why does Hole-in-the-Head appear more often in warm, poorly maintained cichlid tanks?
Hexamita thrives in water with elevated nitrates above 20 ppm and temperatures between 26–30°C/79–86°F, which are standard conditions for most cichlid species. High organic waste from overfeeding creates an environment where the flagellate reproduces rapidly, while nutritional deficiencies from a monotonous diet weaken the immune response that would normally keep parasite loads in check. The lateral line sensory pits are particularly vulnerable because they are open channels in the skin that connect to the nervous system; poor water quality allows bacteria and parasites to colonize these openings. I see the highest caseloads in Oscar and discus tanks where water changes happen monthly instead of weekly and the diet consists almost entirely of freeze-dried foods lacking vitamin C.
Does quarantining new fish prevent Hole-in-the-Head in the main tank?
Quarantine reduces but does not eliminate HITH risk because the syndrome is primarily triggered by chronic environmental stress and nutritional deficiency rather than direct parasite transmission alone. A proper 14-day quarantine at 28°C/82°F with daily observation for stringy white feces, slight head pitting, or lethargy allows early detection of subclinical Hexamita infections before they enter the community tank. However, Spironucleus cysts can survive in water and on equipment, and the parasite is already present in many established cichlid tanks at low, harmless levels. The real prevention comes from maintaining nitrates below 10 ppm, providing a varied diet rich in vitamins, and avoiding activated carbon dust. Quarantine is most valuable for intercepting fish that are already in the early malabsorption phase before they spread the parasite to stressed tank mates.