Hole in the head disease (HITH and HLLE): causes, treatment and the carbon debate

Hole-in-the-head (HITH) is a condition where pits or holes appear in the skin of the head, usually in cichlids such as discus, oscars and angelfish. The most important thing to do is fix the water and the diet, starting with bigger, more frequent water changes to bring nitrate down; mild cases often stop progressing with that alone.

The pits usually start around the eyes and the small sensory pores on the face. When erosion spreads along the lateral line (the row of sensory pores down each side) it's called head and lateral line erosion, or HLLE. Practical Fishkeeping describes them as essentially the same condition: HITH is used more for freshwater fish, HLLE for marine fish like tangs. It can affect many big fish, not only cichlids.

Here's the honest part: nobody has proven a single cause. The leading suspects are:

A gut parasite. Tiny flagellates (single-celled parasites with whip-like tails) called Hexamita and Spironucleus are often found in affected fish. In a 2001 study, Paull and Matthews found Spironucleus vortens in the intestines of all the fish they examined, and in the head lesions of badly affected discus. They proposed it as a possible cause, spreading from the gut. Others argue the parasite takes advantage of fish that are already weakened.
Poor water quality, especially high nitrate from too few water changes. Practical Fishkeeping notes the disease appears more readily in tanks with high nitrate.
Poor or monotonous diet, with possible vitamin and mineral shortfalls.
Stress: crowding, aggression, small tanks for big fish.
Activated carbon (see the debate below).

In practice, most keepers treat it as a combination: a fish weakened by water and diet, with parasites taking advantage.

How to identify it

Small pale pits or enlarged pores on the forehead and around the eyes.
Stringy white or pale poop. This is often linked with gut flagellates, though it has other causes too.
A fish that eats less, spits food out, or slowly loses weight (a hollow belly or "pinched" look behind the head).
Darker colour and hiding.

Easily confused with: injuries from fighting or scraping on decor (one-off wounds, not symmetrical pits); normal sensory pores on large cichlids (small, even, not growing); and ulcers from bacterial infections like columnaris.

When it's a problem

Early: a few small pits or enlarged pores, maybe stringy poop or a fussy appetite. Fix water and diet this week.
Serious: deeper holes, sometimes with white or yellowish material in them; erosion spreading along the lateral line.
Emergency: secondary infections in the open wounds, wasting, and a fish that stops eating. Advanced HITH with wasting has a guarded outlook.
Normal, don't worry: small, even sensory pores on a large cichlid that aren't growing. And during recovery, slow healing is normal: pits can take weeks to months to fill in, and deep ones may leave scars.

What to do right now

Test the water: ammonia, nitrite, nitrate, pH. Nitrate above about 40 mg/L (ppm) is a red flag in a cichlid tank.
Start bigger, more frequent water changes. Aim to get nitrate low and keep it there: for example 30–50% weekly, more if your nitrate is high. Make the change gradual if the tank has been neglected (see pH crash for why).
Look at the poop and the belly. White stringy poop and weight loss point toward internal parasites.
Review the diet. Is it one type of flake or pellet, or old food? Plan to vary it.
Check for bullying and crowding. A stressed fish heals slowly.
Take photos of the lesions so you can track them week by week.

What to do next

Environment first

Every source agrees on this: fix the water and the diet. Large, regular water changes, a varied, good-quality diet (including some vegetable matter for omnivores, and vitamin-rich foods), less crowding, and less aggression.

Metronidazole for gut flagellates

If the fish has signs of internal flagellates (weight loss, white stringy poop, poor appetite), the usual treatment is metronidazole. The Merck Veterinary Manual calls it the only treatment available for spironucleosis, for ornamental fish only, given in food or as a bath if the fish won't eat.

Medicated food works best when the fish is still eating, because the drug goes where the parasite lives: the gut.
Metronidazole is a prescription drug in many countries. Some aquarium products contain it; elsewhere you'll need a vet. Follow the label or your vet's directions; don't guess at doses.
Remove activated carbon during bath treatments.
Metronidazole isn't known to be harsh on shrimp, snails and plants at aquarium doses, but check your product's label for invertebrate warnings.

Courses are commonly repeated over 1–2 weeks. Keep up the water changes and good diet after the medicine ends, or it often comes back.

The activated carbon debate

Activated carbon (black granules in many filter cartridges) has been blamed for HLLE for years. Here's what the evidence actually shows:

Two 2011 studies at US public aquariums kept ocean surgeonfish (a marine tang) with different carbons. Fish exposed to lignite carbon developed HLLE; control fish with no carbon did not. One study also found microscopic lesions in fish exposed to pelleted carbon (summarised here, citing Hemdal and Odum, 2011, and Stamper and others, 2011).
These studies were on one marine species. Nobody has shown the same thing in freshwater cichlids.
Many freshwater HITH cases happen in tanks with no carbon at all.

So experts disagree, and nobody knows for sure. A reasonable middle path: you don't need carbon in most freshwater tanks. Taking it out costs nothing and removes one possible factor. If you use it, rinse off the dust first. But don't expect removing carbon to cure HITH on its own.

Cautions

Don't stack lots of medicines at once on a weak fish.
Plecos, catfish, loaches and scaleless fish: check every product's label for sensitive species.

Products that help

Liquid test kit including nitrate: high nitrate is a key warning sign. Test now and keep checking as you increase water changes.
Water conditioner that removes chlorine: for the larger, more frequent water changes.
Varied, good-quality foods: including vegetable matter for omnivores and vitamin-rich foods. Replace opened food every few months.
Medication or medicated food containing metronidazole: for signs of gut flagellates. Prescription-only in many countries.

Keep on hand before trouble starts: a nitrate test kit and a fresh, varied supply of food. Most HITH starts with what's missing from those two.

How to prevent it

Keep nitrate low with regular, generous water changes. Big cichlids produce lots of waste.
Feed a varied, quality diet and replace opened food every few months.
Give large cichlids a large tank and sensible tankmates.
Quarantine new fish and watch their poop and weight for a few weeks.

Why it happened

Not enough water changes, so nitrate and waste built up.
A narrow diet, or old food that has lost its vitamins.
Big fish in a tank too small, or constant aggression.
Internal parasites present at low levels, which flare when the fish is stressed.
Possibly carbon dust, especially in marine tanks.

When to get help

See a vet who treats fish if the fish stops eating, if holes deepen despite clean water, or if you need metronidazole and can't get it legally without a prescription. A vet can check a fecal sample for flagellates. A good local cichlid or discus keeper can be a great second opinion on diet and tank setup. Honest limits: advanced HITH with wasting has a guarded outlook, and scars may stay even after the fish recovers.

In the game

In Multiple Tank Syndrome, the Hole-in-the-head card is "pits in the head from poor water and a gut parasite": cichlids and big fish (6 inches and up) in the hit tank die. The Metronidazole card or the Medicine basket saves the tank. In real life, the water changes matter as much as the medicine.