Internal parasites in fish: Camallanus and Hexamita — diagnosis and treatment
Red worms protruding from the vent (Camallanus) or white stringy faeces and 'hole-in-the-head' in discus (Hexamita) — two different parasites needing different drugs: levamisole and metronidazole. Detailed treatment protocol.
Internal parasites are an invisible but common problem, especially in livebearers and cichlids. The two main diagnoses are Camallanus cotti (a nematode) and Hexamita / Spironucleus (a flagellate protist). Symptoms look like 'general malaise' and are often missed; both, however, respond well to treatment if correctly identified.
Camallanus: telltale sign — thin red worms 5–15 mm protruding from the vent, most often in guppies, mollies, angelfish. Spread via copepods or new carriers; untreated they cause wasting, stunted growth, in severe cases death. Hexamita: white, mucous, stringy faeces, the fish eats but loses weight; in discus and Malawi cichlids — the classic 'hole-in-the-head' (pits in the lateral-line skin glands).
Treatment
Camallanus: levamisole hydrochloride 1 g per 100 L (or 2 mg/L), 24 hours in full darkness (do not feed), then a 50% water change; repeat after 7 days. Run a third course at day 14 for safety. In parallel — siphon the substrate and remove expelled worms. Hexamita: metronidazole 250 mg per 100 L in the water + in food at 1% (5 g of drug per 500 g of food) for 7–10 days.
Prevention and prognosis: with a proper course, recovery is 90%+. Prevention — 3–4 weeks of quarantine for every new fish, varied quality food (live/frozen only from trusted sources). Discus prone to Hexamita benefit from a preventive metronidazole-in-food course every six months.
FAQ
- What if every guppy from a shop batch has Camallanus?
- Quarantine is mandatory. Treat the whole batch with levamisole through the full schedule before moving them to the main tank. Pregnant females can be treated — the drug is safe for developing fry.
- Can I use human metronidazole (e.g., Metrogyl) for fish?
- Yes, the active ingredient is the same. Just calculate the dose by weight and volume — human tablets are 250–500 mg; for an aquarium, 250 mg is dissolved per 100 L.
- Are Camallanus and Hexamita contagious to other tank inhabitants?
- Camallanus easily transfers between all freshwater fish. Hexamita mainly affects cichlids and anabantoids. Treat the whole tank, not just the visibly sick fish.
Scientific board — ichthyologists and veterinarians
Ichthyologists and veterinarians with university degrees · Reference FishBase, Seriously Fish and peer-reviewed literature · Sign every reviewed article with their credentials shown
Goldie editorial team
Practising aquarists with a combined 30+ years of experience · Biologists and editors, fact-checking against FishBase and Seriously Fish · Every piece is reviewed by a qualified ichthyologist before publication
Sources
- Seriously Fish — Camallanus · Seriously Fish · 2026-05-31
- Practical Fishkeeping — Hexamita · Practical Fishkeeping · 2026-05-31
- WHO — Levamisole pharmacology · WHO · 2026-05-31