Metronidazole in Aquariums: Treating Hexamita and Anaerobic Infections
When a cichlid develops pitting lesions on its head, or a discus stops eating and passes white, stringy faeces, you are almost certainly dealing with an internal protozoan infection. Metronidazole in aquariums is the treatment of choice for these conditions — it is effective, relatively safe for fish when used correctly, and available in Singapore through online marketplaces like Shopee and specialist aquarium retailers. This guide from Gensou Aquascaping at 5 Everton Park, Singapore covers when to use metronidazole, how to dose it correctly, and what to watch for during treatment.
What Metronidazole Treats
Metronidazole (also sold as Flagyl in pharmaceutical contexts) is a nitroimidazole antibiotic and antiprotozoal drug. In freshwater aquariums, it is primarily used to treat:
- Hexamita — an internal flagellate protozoan that infects the intestinal tract of cichlids, discus, and other fish
- Hole-in-the-head disease (HITH) — sensory pit erosion most commonly seen on oscars, green terrors, and discus; often linked to Hexamita combined with nutritional deficiency
- Anaerobic bacterial infections — metronidazole is active against gram-negative anaerobes
- Spironucleus — a related flagellate that causes bloating, lethargy, and whitish faeces
It is not effective against external parasites like ich, velvet, or fish lice, nor against most aerobic bacteria. Correct diagnosis matters — using metronidazole against the wrong condition wastes time when a fish is declining.
Recognising Hexamita and HITH
Early Hexamita infection presents as loss of appetite, lethargy, pale or dark colouration changes, and white or grey stringy faeces. As infection progresses, pitting or open lesions develop around the lateral line and head — the hallmark of hole-in-the-head disease. Discus keepers will sometimes see early signs as subtle darkening and clamped fins before any visible lesions appear. The sooner you act, the better the outcome — HITH lesions, once deep, leave permanent scarring even after successful treatment.
Dosing Metronidazole in the Aquarium
The most effective method for internal protozoan infections is food-soaking: dissolve metronidazole in a small amount of water and soak the fish’s food before feeding. This delivers the medication directly to the gut where Hexamita resides, and avoids exposing your biological filter to drug levels that can slow nitrification.
Standard dosing via water column: 250 to 500 mg per 40 litres of tank water, every 48 hours for three to five treatments. If the fish is eating, the food-soak method at 5 mg per gram of food is more targeted. Turn off UV sterilisers and activated carbon filtration during treatment — both will degrade or remove the medication before it can act.
Effect on Biological Filtration
Metronidazole is generally considered to have minimal impact on beneficial nitrifying bacteria (Nitrosomonas and Nitrospira) at therapeutic doses, unlike broad-spectrum antibiotics such as erythromycin. That said, it is prudent to monitor ammonia and nitrite during treatment, particularly in heavily stocked tanks. Performing a 30% water change 48 hours after each dose — just before re-dosing — removes residual medication and reduces any cumulative impact on the biological filter.
Sourcing Metronidazole in Singapore
Pharmaceutical-grade metronidazole tablets (200 mg or 400 mg) can be found at some Guardian and Watsons pharmacies, or through online listings on Shopee and Lazada. Fish-specific formulations such as Seachem Metronidazole (powder) are available at well-stocked aquarium retailers and provide precise dosing for aquarium use. Seachem’s formulation is widely preferred by hobbyists because it is pure, consistent, and comes with clear aquarium-specific instructions. Expect to pay $15 to $30 for a 5 gram tub, which provides multiple treatment courses for a small tank.
Combining With Other Medications
Hole-in-the-head disease and advanced Hexamita infections often benefit from a combination approach. Metronidazole is frequently paired with an antibiotic like kanamycin (Seachem Kanaplex) to address any secondary bacterial infection in the lesions. This combination is safe for most tropical fish at therapeutic doses. Do not combine metronidazole with salt-based treatments — the combination can increase osmotic stress without improving antiprotozoal efficacy.
Nutritional deficiency — particularly insufficient vitamin C and trace minerals — is thought to contribute to HITH susceptibility. After completing the metronidazole treatment guide protocol, switch to a high-quality, varied diet. For cichlids and discus, spirulina-enhanced flakes, frozen mysis shrimp, and live blackworms help restore condition and reduce recurrence risk.
When Treatment Fails
If symptoms do not improve after two full treatment courses, reconsider the diagnosis. Persistent white faeces with no lesion development may point to intestinal worms (treatable with praziquantel or fenbendazole) rather than Hexamita. A fish that continues to decline despite treatment likely has concurrent organ damage or a secondary infection requiring a different antibiotic. At this point, consultation with an experienced aquarist or veterinarian familiar with fish medicine is the most pragmatic step. Gensou Aquascaping has worked with discus and cichlid keepers across Singapore through difficult disease management cases — get in touch if you need tailored advice for your specific situation.
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emilynakatani
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