How to Treat Internal Parasites in Freshwater Fish
A fish that eats eagerly but steadily loses weight, passes white stringy faeces, or shows a visibly bloated or hollow belly is displaying the classic warning signs of an internal parasitic infection. Knowing how to treat internal parasites in freshwater fish correctly — using the right medication at the right dose — separates a full recovery from a prolonged decline. This guide from Gensou Aquascaping at 5 Everton Park, Singapore covers identification, treatment protocols, and prevention strategies suited to our local conditions.
Recognising the Signs of Internal Parasites
Internal parasites are frustrating to diagnose because the visible symptoms overlap with several other conditions, including bacterial infections and poor nutrition. The most reliable indicators are white, stringy, or mucus-like faeces; a fish that eats normally but loses body mass over weeks; a swollen abdomen without raised scales (which would suggest dropsy); and increased lethargy paired with occasional flashing or rubbing against surfaces.
Wasting disease — a visibly pinched belly, sunken flanks, and prominent spine — is a particularly telling sign in livebearers and cichlids. Camallanus worms, one of the most common internal parasites in the hobby, are identifiable by their distinctive red, thread-like protrusion from the fish’s vent. If you see this, treatment is urgent.
Common Internal Parasites in Freshwater Fish
Camallanus nematodes are widespread in Singapore fish stocks, often introduced via wild-caught fish or poorly quarantined imports. They infect the intestinal tract and can spread rapidly through a community tank when fish consume the larvae shed by infected individuals. Hexamita (the parasite behind hole-in-the-head disease) primarily affects cichlids, particularly oscars and discus, presenting as pitting on the head and lateral line. Capillaria causes wasting and white faeces in many species.
Microsporidian parasites such as Pleistophora hyphessobryconis — the agent of neon tetra disease — cause pale patches and muscle deterioration in small tetras. Unfortunately, these have no known effective treatment and affected fish should be humanely culled to prevent spread.
Setting Up a Quarantine Tank
Before treating, move visibly infected fish to a quarantine tank — a bare-bottomed 20–40 litre container with a small sponge filter, a hiding spot, and a heater set to match your display tank’s temperature. Treating in a quarantine tank protects your display tank’s biological filtration from medication, allows you to monitor individual fish responses, and prevents medication from affecting invertebrates or sensitive tankmates.
If Camallanus worms have been confirmed, treat the entire display tank regardless — larvae will already be present in the water column and substrate, and untreated fish will be re-infected. In this scenario, remove live plants and invertebrates if possible, as many medications are harmful to shrimp.
Choosing the Right Medication
Levamisole is the most effective treatment for Camallanus and most nematode infections. It paralyses the worms, which are then expelled by the fish. A standard dose is 10 mg per litre as a bath treatment, with the fish left in the medicated water for 24–48 hours. The dead and expelled worms must be siphoned out promptly — if left to decompose, they can release larvae or cause an ammonia spike. A second treatment 21 days later catches any larvae that hatched after the first dose.
Metronidazole targets protozoan and flagellate parasites, including Hexamita. It is most effective when incorporated into food — mix metronidazole powder at roughly 1% by weight into a paste with gel food or soaked pellets. Fish that are still eating will absorb the medication directly where it’s needed. If the fish has stopped eating, a water bath treatment at 250–500 mg per 40 litres for 72 hours is an alternative, though absorption via the gills is less targeted.
Fenbendazole for Stubborn Worm Infections
When levamisole is difficult to source, fenbendazole (found in some commercial deworming products) is a viable alternative for nematode infections. Dose at approximately 2 mg per litre, added to the tank water. Unlike levamisole, fenbendazole kills rather than paralyses worms, so expelled worms are already dead and slightly easier to manage, though siphoning is still essential.
Note that fenbendazole can harm snails and some invertebrates at therapeutic doses. If your display tank houses nerite snails or shrimp, relocate them before treatment. Most local aquarium shops around Serangoon North stock both levamisole-based treatments and metronidazole — ask specifically for the active ingredient rather than a brand name, as product availability changes frequently.
Supportive Care During Treatment
Medicated fish are under physiological stress. During treatment, maintain excellent water quality through daily partial water changes of 20–25%, increase surface agitation slightly to maximise dissolved oxygen, and reduce feeding to once daily with easily digestible foods. Adding a small amount of aquarium salt (1–2 g per litre in the quarantine tank) can support the fish’s electrolyte balance and reduce osmotic stress — particularly helpful for cichlids and tetras during recovery.
Watch for secondary bacterial infections. Fish weakened by parasites often develop fin rot or opportunistic ulcers. A short course of a broad-spectrum antibacterial (such as kanamycin) may be appropriate if you see wound sites opening, but introduce it only after confirming the parasite treatment is complete to avoid unnecessary medication stacking.
Quarantine as Your Best Prevention
Most internal parasite introductions are preventable. A strict quarantine period of at least three to four weeks for all new fish — including those from reputable local shops — dramatically reduces risk. During quarantine, watch for the behavioural signs described above and consider a prophylactic levamisole treatment for any wild-caught species or fish from mixed display tanks.
Group buys and Carousell purchases carry higher parasite risk than purpose-bred fish from clean facilities, simply due to the unknown health history of the sellers’ tanks. This is not a reason to avoid them, but it makes quarantine even more important. The team at Gensou Aquascaping is always happy to advise on quarantine protocols that fit your setup and species mix.
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