Hexamita and Hole-in-the-Head in discus: complete Metronidazole treatment

In short

• Cause: Spironucleus vortens (historically called "Hexamita") — an opportunistic protozoan present latently in most discus.
• Symptom #1: stringy white feces + gradual food refusal.
• Advanced stage: Hole-in-the-Head (HITH) — pits and erosions on the head and lateral line.
• Standard treatment: Metronidazole 400 mg / 100 L (26 US gal), 10 days, remove the carbon, temperature 30°C (86°F).
• More effective: medicated food (1 g Metronidazole / 100 g food) if the fish is still eating.
• Do NOT treat only the sick fish — every discus in the tank carries the parasite. Treat the whole aquarium.
• Prevention: temperature 29–30°C (84–86°F), a varied diet, a 4-week quarantine.

What you'll learn from this guide

Hexamita in discus (Symphysodon) is the most common internal parasitic disease — and also the most often misdiagnosed and treated too late. The parasite Spironucleus vortens lives latently in the gut of most discus and only turns pathogenic when immunity drops: temperature below 28°C (82°F), transport stress, unstable parameters or a monotonous diet. This guide shows you how to recognize the early signs (stringy white feces), how to apply the Metronidazole 400 mg/100L protocol step by step, how to prepare medicated food (more effective than a bath), what alternatives exist (JBL Spirohexol, eSHa Hexamita) and how to prevent a recurrence.

What Hexamita is — a short biology

Key idea: the parasite is constantly present in discus — you don't "catch" it from another aquarium, you already have it. It only becomes a problem when the immune system gives way.

Triggers — what activates Hexamita

Almost always the real problem is the environment, not the parasite. See the guide on water parameters for beginners.

Symptoms — visual diagnosis

Early stage (days 1–7)

• ✅ White, transparent, stringy feces — symptom #1, see "Discus has white feces"
• ✅ Fussy appetite → gradual food refusal (see "Discus not eating")
• ✅ Slightly faded colors
• ✅ Isolation from the group
• ✅ Swimming a little "wobbly" or with a clamped tail

Advanced stage (weeks 2–4)

• ❌ Visible weight loss — a sunken belly, a pronounced "spine line"
• ❌ Pits and erosions on the head = Hole-in-the-Head
• ❌ Erosions along the lateral line
• ❌ Dark, washed-out colors
• ❌ Extreme lethargy, sitting in a corner
• ❌ Excess white mucus on the skin

HITH appears in the advanced stage — secondary bacteria invade the areas weakened by Hexamita and produce the classic "hole in the head" lesions.

Standard treatment — Metronidazole in a medicated bath

Metronidazole is the global standard antiprotozoal antibiotic for Hexamita. It's found in:

Exact dosage

• 400 mg Metronidazole / 100 L (26 US gal) of water
• Treat the whole aquarium
• Duration: 10 consecutive days
• Temperature: 30°C (86°F) (speeds up the effect, no higher)

🧮 For your tank's real volume (minus substrate and decor), use the treatment calculator — avoid overdosing.

Daily schedule (10 days)

Mandatory rules during treatment

• ✅ Remove the activated carbon — it fully absorbs the Metronidazole (cancels the treatment)
• ✅ Max aeration — Metronidazole lowers oxygen
• ✅ Dim or off lights — Metronidazole is light-sensitive
• ✅ Temperature 30°C (86°F) constant
• ❌ Do NOT add other medications at the same time
• ❌ Do NOT overfeed — sick fish eat little, the surplus rots

Alternative treatment — medicated food (more effective)

If the fish is still eating, medicated food delivers the active straight into the gut — where the parasite lives. 3–5× more effective than a bath.

The recipe (for 100 g of food)

Preparation (10 minutes)

1. Crush the Metronidazole tablets into a fine powder (or use veterinary powder directly).
2. Mix the powder into the beef heart or into pellets soaked with 5 ml of water.
3. Add the crushed garlic and the multivitamin.
4. Form small pellets (1 cm / 0.4 in) and freeze them in a bag.
5. Thaw one for each meal.

Feeding schedule

• 2–3 pieces / fish / day
• 10 consecutive days
• 2 meals a day (morning + evening)
• The water stays untreated — no Metronidazole in the aquarium

⚠️ If some fish don't eat, combine: medicated food for those still eating + the classic Metronidazole bath for all of them.

Advanced HITH treatment — extra steps

If pits have already appeared on the head:

1. Metronidazole — the standard protocol (bath + food) for 10 days.
2. 30% water changes every 2 days with fresh water at 30°C (86°F).
3. A high-protein diet + vitamins — multivitamin in every meal.
4. Indian almond (catappa) leaves (1 leaf / 50 L / 13 US gal) — tannins reduce secondary bacteria and aid healing.
5. A UV unit (if you have one) — reduces the bacterial load in the water.
6. After the 10 days → a second round of Metronidazole after a 7-day break.

Recovery: 3–6 weeks after treatment. Small pits close; large ones may leave permanent scars (but the fish stays healthy).

Alternatives to Metronidazole — EU products

Dimetridazole is stronger but harder to find in the EU — banned in some countries for food fish.

Common mistakes

❌ Stopping treatment after 3–4 days "because the fish seems better" → Hexamita comes back aggressively in 7–10 days.

❌ Treating only the sick fish in a hospital tank → the rest of the discus reactivate the parasite in 1–2 months. Treat the whole aquarium.

❌ Mixing Metronidazole with other medications → toxicity, and the parasite becomes resistant.

❌ Low temperature during treatment → Hexamita thrives below 28°C (82°F), and the treatment becomes ineffective.

❌ Ignoring white feces "because the fish is still eating" → in 2–3 weeks you reach HITH.

❌ Forgotten activated carbon in the filter → zero treatment, the Metronidazole is fully absorbed within 24h.

❌ A single dose with no water changes → the Metronidazole metabolites become toxic after days 5–6.

Prevention — how to avoid a recurrence

See the guides on how to test discus water and simple quarantine for beginners.

Differentiation — Hexamita vs other causes of white feces

If you're not sure, use the visual diagnosis.

When to consult a specialist

• A full 10-day treatment gives no results → possibly a resistant strain, or a misdiagnosis
• HITH deepens after week 2 → possibly a serious secondary bacterial infection
• Mass mortality at once → rule out other causes (poor parameters, poisoning)
• Affected juveniles → difficult treatment, adjusted doses

Post on dedicated forums with photos + water parameters + the protocol you followed — or contact an experienced breeder from the discus.ro directory.

Final checklist — complete Hexamita treatment

• ☐ Confirmed the diagnosis (stringy white feces + food refusal)
• ☐ Tested ammonia, nitrite, nitrate (ruled out poor parameters)
• ☐ Removed the activated carbon from the filter
• ☐ Raised the temperature to 30°C (86°F) constant
• ☐ Max aeration
• ☐ Calculated the exact dose for the net aquarium volume
• ☐ The 10-day schedule with 25% changes + re-dosing
• ☐ (Optional) medicated food prepared and frozen
• ☐ Multivitamin and garlic in the food
• ☐ Post-treatment plan: fresh carbon + large changes + multivitamin for 2 weeks
• ☐ Investigated the cause that triggered the disease (temperature? stress? parameters?)

What to do next

📚 Continue learning:

• Discus has white feces — the full differential diagnosis
• Internal worms in discus — Camallanus, Capillaria, a frequent co-infection
• Post-transport stress — the main activator of latent Hexamita
• Constipation / Swim Bladder — telling it apart from abnormal feces
• The aquarist's pharmacy — Metronidazole in your kit

🔧 Practical resources:

• Treatment dose calculator — Metronidazole, Levamisole, Praziquantel
• Visual diagnosis — at the first symptoms
• Aquarium treatments — shop — Seachem MetroPlex, JBL, eSHa

🐠 Final tip: Hexamita isn't a death sentence — the right protocol saves 90% of cases caught in time. White feces are an alarm, not a verdict. Act immediately.