Camallanus Worms in Aquarium Fish: An Evidence-Based Guide
- Macauley Sykes
- 1 day ago
- 14 min read
Updated: 34 minutes ago
Camallanus worms are among the most recognisable internal parasites in aquarium fish.
Those thin red threads protruding from a fish’s vent are instantly worrying, and online discussions often make them sound catastrophic. In reality, these nematodes are straightforward to eliminate when treated correctly.
Infestations are fairly common in community aquaria, particularly when new fish are introduced without proper quarantine measures. They are also frequently seen in pufferfish, which are often imported from mixed sources and can pick up parasites during transit or storage. Surveys of ornamental fish imports consistently rank Camallanus cotti among the most prevalent nematodes found in tropical species, particularly livebearers, barbs, and cichlids.
This guide outlines an evidence-based treatment protocol for Camallanus worms, using Levamisole Hydrochloride (HCl). The method is drawn from veterinary parasitology sources, university aquaculture research, and extensive hands-on experience in treating these parasites in pufferfish and other aquarium species.
Understanding Camallanus: The Life Cycle
Understanding the life cycle of Camallanus is essential for effective treatment and prevention. These nematodes have a flexible cycle that can proceed with or without intermediate hosts, depending on species and environmental conditions.
1. Adult worms in the fish
Adult males and females live in the intestinal tract of the host fish.
The worms attach to the gut wall using a buccal capsule and feed on blood and tissue fluids.
Mature females are ovoviviparous: they hatch the larvae internally and then release live first-stage larvae (L1) directly into the water through the fish’s vent.
2. Free-living larval stage
Once released, the L1 larvae drift in the water column or settle into the substrate and filter debris.
At tropical aquarium temperatures (26–28 °C), they remain viable for several days to about three weeks, depending on temperature, oxygen, and organic matter.
These larvae cannot reproduce outside the host but can survive long enough to infect another organism.
3. Intermediate host stage (optional but common)
The free L1 larvae are often eaten by tiny aquatic crustaceans such as copepods, ostracods, or amphipods.
Inside these hosts, the larvae develop to the third-stage (L3) over about two to three weeks.
L3 is the infective stage: the point at which the parasite can resume its life cycle if the crustacean is eaten by a fish.
Snails do not serve as intermediate hosts. There is no evidence that Camallanus larvae develop or encyst within molluscs.
4. Infection of new fish
When a fish eats an infected crustacean or swallows free-swimming larvae directly from the water or substrate, the L3 larvae are released into the intestine.
They penetrate the intestinal lining and feed on host fluids while maturing into adults.
This development takes about three to four weeks at tropical temperatures.
5. Reproduction and continuation
Once mature, adult females begin releasing new L1 larvae, restarting the cycle.
Because the larvae are produced continuously, infections can persist indefinitely unless treated.
There are no dormant eggs or cysts that lie waiting in the gravel, but larvae or infected crustaceans can survive long enough to cause reinfection if not removed.
Symptoms
Camallanus infections are easy to recognise once they reach maturity.
The most obvious sign is one or more thin red worms protruding from the fish’s vent.
Other signs include:
Weight loss despite normal or increased appetite
Darker or duller colouration
Stringy, reddish faeces
Reduced vitality or slower feeding
Because worms become visible only at the adult stage, the infection has usually been developing for several weeks. Prompt treatment is essential but straightforward.
Why Panic Isn’t Necessary
It’s easy to understand why people panic when they first see Camallanus. The sight of worms hanging from a fish’s vent is unpleasant, and the internet is full of alarmist advice suggesting you must put the fish in a hospital tank, strip the display aquarium, discard the substrate, or sterilise everything in sight.
None of that is necessary. The key to success is consistent, properly timed treatment using the correct medication and dosage.
As I often tell new keepers:
“Don’t worry. A lot of the stuff you’ll read about Camallanus worms online is simply alarmist. It mixes general nematode facts with outdated hobby lore and then applies them to Camallanus incorrectly. They’re actually easily beaten with consistent treatment and proper re-dosing intervals with the right medication. No need to panic, rip apart, or nuke the tank.”
The Evidence-Based Treatment Protocol
The treatment of choice for Camallanus worms is Levamisole Hydrochloride (HCl).
Levamisole is a selective nematocide that acts as a cholinergic agonist, disrupting the worms’ nerve-signal transmission. It binds to nicotinic acetylcholine receptors in the nematode’s muscle cells, causing continuous stimulation of their neuromuscular system. This overstimulation quickly leads to spastic paralysis and, ultimately, complete loss of movement.
In short, Levamisole paralyses the worms.
Once paralysed, the worms can no longer maintain their grip on the intestinal wall and are expelled naturally through the vent within about 12 to 24 hours. Levamisole does not dissolve or chemically damage the parasites; it simply forces them to release their hold.
Fish readily absorb Levamisole from the surrounding water through the gills and, to a lesser extent, through the skin. After absorption, the drug enters the bloodstream and circulates rapidly throughout the body. This systemic absorption is essential for eliminating internal nematodes such as Camallanus, which feed on blood and tissue fluids deep within the intestine. Because the medication reaches the parasite via the fish’s circulatory system, it can act on worms that are not in direct contact with the intestinal lumen.
Levamisole affects only specific stages of the parasite’s life cycle. It kills adult worms and late-stage juveniles feeding inside the intestine but does not affect early larvae free in the water, substrate, or filter, nor those protected within intermediate hosts such as copepods or ostracods. For this reason, repeated treatments are essential. Each new dose removes the next generation of worms that have matured since the previous round, gradually eradicating them from the system.
When used correctly, Levamisole is among the safest and most effective treatments for intestinal nematodes in ornamental fish. It clears active infections without harming plants, the biological filter, or most invertebrates, and has been proven reliable through decades of veterinary and aquaculture use. The medication is fast-acting, well-tolerated, and exceptionally effective against nematodes.
Fish-Specific Products:
When using proprietary aquarium medications containing Levamisole Hydrochloride (HCl), always follow the manufacturer’s instructions exactly as stated on the label. These products are formulated, tested, and approved under veterinary and biocidal regulations to ensure both efficacy and safety when used according to their own directions.
Manufacturer dosing schedules account for differences in formulation strength, drop size, stability, and safety margins, and these parameters can vary between brands and regions. For that reason, the dosage printed on the product packaging is the only one that should be used.
Independent veterinary and aquaculture research demonstrates that Levamisole HCl is effective and well-tolerated in the range of approximately 2–3 mg/L, with toxicity generally not observed until levels exceed 5–10 mg/L in most freshwater species.
This means aquarists can follow the following dosing instructions with confidence, knowing that they correspond closely to the concentrations validated by scientific research.
eSHa NDX (UK / EU):
A broad-spectrum anti-helminthic formulated specifically for ornamental freshwater and marine fish, eSHa NDX is designed for the control and eradication of intestinal nematodes.
Its drop-based dosing system offers simplicity and accuracy, allowing precise control even in small treatment volumes. This has made eSHa NDX a popular choice among aquarists and professional breeders across the UK and EU for the treatment of confirmed nematode infestations. Active ingredient: Levamisole Hydrochloride (HCl) – 5.4 % w/v (54 mg per mL)
Dosage:
1 'drop' per litre of aquarium water.
50 litres = 50 drops
100 litres = 100 drops
200 litres = 200 drops
Conversion (Drops → Millilitres):
Manufacturer data: 100 drops = 4.4 mL
Therefore, 1 drop ≈ 0.044 mL
50 drops = 2.2 mL
100 drops = 4.4 mL
200 drops = 8.8 mL
This produces an approximate concentration of 2.38 mg/L Levamisole HCl, which falls squarely within the scientifically recognised therapeutic range of 2–3 mg/L. The formulation and drop size have been validated by eSHa for both safety and efficacy under EU and UK regulatory testing.
Fritz Expel-P (USA / Canada)
A broad-spectrum anti-helminthic formulated specifically for ornamental freshwater and marine fish, Fritz Expel-P is designed for the control and elimination of intestinal nematodes.
Its pre-measured powdered format ensures consistent, easy dosing and rapid solubility, making it ideal for both small and large aquaria. Trusted by aquarists and breeders across North America, Expel-P provides an effective treatment for confirmed nematode infestations.
Active ingredient: Levamisole Hydrochloride (HCl) – 113 mg per packet (0.113 g)
Dosage:
Use 1 packet per 10 US gallons (≈ 38 litres) of aquarium water.
For larger systems, scale proportionally:
2 packets per 20 US gallons (≈ 76 L)
4 packets per 40 US gallons (≈ 152 L)
8 packets per 80 US gallons (≈ 304 L)
This dosage produces an approximate concentration of 2.97 mg/L Levamisole HCl, which lies comfortably within the scientifically recognised therapeutic range of 2–3 mg/L.
Packet strength and dosage are as specified by Fritz Aquatics for ornamental aquarium use across the USA and Canada.
Harker's Harka-Verm (UK / Ireland)
A practical and cost-effective source of Levamisole Hydrochloride in the UK and Ireland is Harka-Verm, a wormer produced by Harkers for pigeons and cage birds.
Although marketed for birds, it contains the same active ingredient used by fish veterinarians for nematode control in ornamental fish.
Harka-Verm comes in a handy bottle with a cap that accepts the nozzle of the included dosing syringe, allowing accurate measurement and clean dispensing. It is widely used by aquarists in the UK and Ireland because it is highly concentrated, stable, and inexpensive. Typical UK pricing is about £10–£15 per 100 ml (Oct 2025). Each 100 mL bottle contains 8,000 mg of Levamisole HCl, enough to treat around 4,000 litres of aquarium water, making it one of the most economical and accessible sources of pure Levamisole available to aquarists.
Contains 8% Levamisole Hydrochloride (80 mg per mL).
Dosage (for 2.4 mg/L Levamisole HCl):
0.03 mL per litre of aquarium water.
3 mL per 100 litres of aquarium water.
6 mL per 200 litres of aquarium water.
This delivers an approximate concentration of 2.4 mg/L Levamisole HCl, which lies comfortably within the scientifically recognised therapeutic range of 2–3 mg/L. This slightly rounded dosage simplifies measurement without compromising safety or efficacy, aligning with established veterinary data on Levamisole HCL use in ornamental fish.
Treatment Recommendations
Day 1
Turn off the lights (Levamisole degrades rapidly in light)
Remove carbon, resins (like Purigen), and turn off UV clarifiers/sterilisers if fitted.
Dose the aquarium using your choice of suitable medication.
Keep the lights off for 24 hours.
Ensure good surface agitation or aeration.
Day 2
After 24 hours, perform a 70–80% water change.
Vacuum the substrate thoroughly to remove expelled worms and faeces.
Repeat treatments
At tropical aquarium temperatures (26–28 °C), Camallanus worms mature from larvae to adults in roughly 3–4 weeks. Levamisole only kills the feeding stages inside the fish, so repeating the treatment at the right interval is key to eliminating each new generation before it reproduces.
Repeat the full treatment after 2–3 weeks. This interval allows the larvae that survived the first dose and matured inside the fish to become susceptible to the Levamisole treatment.
Complete two to three total treatments to ensure the life cycle is broken completely.
In warm, heavily stocked systems where development may be faster, dosing every 7–10 days is acceptable as a precaution, but not essential.
Temperature
Within normal tropical aquarium temperatures (24–30 °C), Levamisole Hydrochloride remains fully effective. Efficacy may decrease slightly below about 22 °C because fish metabolism and worm activity both slow, reducing uptake.
Heat above 32 °C can accelerate chemical breakdown.
Maintaining the tank between 26 °C and 28 °C during treatment provides optimal results.
Praziquantel and Levamisole: Understanding the Difference
Medications which utilise Praziquantel (such as PraziPro) are one of the gold-standard treatments in fish medicine. It is highly effective against cestodes (tapeworms) and trematodes (flukes), both internal and external, by damaging their surface membranes and disrupting calcium ion channels.
However, Praziquantel is not a nematocide. It has little or no effect on roundworms such as Camallanus.
Levamisole is specifically designed for nematodes. It acts on their nervous system, causing complete paralysis, after which the fish expels them naturally. For this reason, Levamisole remains the definitive choice for Camallanus infections, while Praziquantel remains the benchmark for flukes and tapeworms. Each has its place; knowing which to use is what ensures complete control and prevents unnecessary medication overlap.
Prevention
Quarantine new fish for at least 4–6 weeks before adding them to display tanks.
Avoid feeding wild-collected live foods, which can carry crustacean hosts.
Rinse or freeze live foods from commercial suppliers before use.
Maintain strong filtration and regular substrate cleaning to remove organic waste.
Addressing Misinformation in the Hobby
While most aquarists share advice with good intentions, some widely circulated online articles have repeated outdated or inaccurate information about Camallanus worms and Levamisole treatment. These errors often sound convincing, but they can lead to failed treatments and unnecessary livestock losses. The intention here is not to criticise individuals but to encourage a higher standard of accuracy and evidence within the aquarium community. Aquarium medicine deserves the same care and scrutiny as any other field of animal health. When misinformation is replaced with verified parasitology, aquaculture data, and real-world observation, success rates improve dramatically and unnecessary livestock suffering is reduced.
One common mistake is the claim that Camallanus lays "eggs" that hatch in the aquarium. This is scientifically incorrect. The genus is viviparous, meaning females release live larvae, not eggs. These larvae are often consumed by copepods or other crustaceans, where they continue to develop before becoming infectious again. Misunderstanding this process leads to poor treatment timing, since Levamisole only affects adult and larval worms inside the fish at the time of dosing, not free-living or intermediate stages in the environment. This is why repeating treatment after 14–21 days is essential: it targets newly matured worms that were not yet parasitic during the first dose.
Another widespread misunderstanding is the claim that Levamisole must be administered only through medicated food, and that adding it directly to the aquarium is ineffective.
This belief conflicts with decades of veterinary and aquaculture research. Levamisole Hydrochloride is readily absorbed through the gills and skin, entering systemic circulation within hours of being added to the aquarium water. Controlled studies have demonstrated measurable plasma concentrations after dosing at around 2 mg/L—levels sufficient to paralyse intestinal nematodes such as Camallanus cotti within a 24-hour treatment window.
Feeding medicated food can be useful in commercial aquaculture, where precise dosing and uniform feed intake can be guaranteed, but it is impractical or unreliable in many ornamental cases, particularly when fish are anorexic, stressed, or housed in mixed species tanks. Water-column dosing ensures that every fish receives an equal systemic dose, regardless of appetite.
The evidence therefore overwhelmingly supports direct aquarium dosing as the most reliable and universally effective method, particularly in community and quarantine systems.
Another recurring issue is inaccurate dosing advice. Some online sources promote excessive or arbitrary Levamisole doses, sometimes several times higher than those supported by veterinary and aquaculture literature. The evidence-based therapeutic concentration for ornamental fish is around 2 mg/L of Levamisole Hydrochloride for 24 hours, followed by a large water change and substrate cleaning. Higher doses have not demonstrated better efficacy and only increase the risk of stress or toxicity in sensitive species such as puffers, loaches, and shrimp.
A further misconception is that Levamisole is a broad-spectrum antiparasitic. In reality, it is a nematocide and does not affect flatworms, tapeworms, or protozoan parasites. These groups require different compounds. Using the wrong treatment wastes time and may allow infections to worsen.
Equally misleading are claims that garlic, herbal tonics, or salt can eradicate internal worms. There is no peer-reviewed evidence that such substances are effective against internal nematodes in fish. Published studies showing antiparasitic activity of garlic apply primarily to external monogeneans, not to intestinal worms such as Camallanus. While some additives may improve appetite or mild immune response, they do not substitute for targeted anthelmintic treatment.
Another area of confusion concerns the purpose of repeat dosing. Some sources claim that second or third treatments are intended to “destroy eggs” or “clear the filter of eggs.” This is inaccurate because Camallanus does not produce eggs that persist in the aquarium. The repeat treatment is designed to eliminate larvae that were not yet infective or parasitic during the first round, ensuring that the parasite’s life cycle is fully broken.
Finally, some aquarists are advised to tear down and sterilise the entire aquarium after discovering Camallanus. This approach is unnecessary and often harmful. Camallanus larvae are not resilient free-living organisms that persist indefinitely in water or substrate. Once infected fish have been treated and expelled worms are removed, the parasite’s life cycle collapses in the absence of a host. A thorough substrate siphon, filter clean in tank water, and a substantial water change are entirely sufficient.
Author’s Note
This guide is written to help aquarists treat Camallanus confidently and without panic.
The dosing protocols and recommendations are based on veterinary parasitology research and many years of direct aquarium treatment, including work with pufferfish systems where Camallanus is particularly common.
The goal is simple: calm, evidence-based guidance that empowers fishkeepers to act decisively, safely, and without fear.
Disclaimer
The information in this guide is provided for educational purposes and is based on published veterinary and aquaculture research as well as extensive practical experience in ornamental fishkeeping. It is not a substitute for professional veterinary advice.
Always verify product concentrations and calculate doses carefully for your own aquarium volume. If you are uncertain or if fish show severe symptoms, consult a qualified aquatic veterinarian before beginning treatment. The author and publisher accept no responsibility for misuse of medications or deviation from manufacturer instructions.
References & Further Reading
Primary research on Camallanus biology and life cycle
Levsen, A. (2001) ‘Transmission ecology and larval behaviour of Camallanus cotti under aquarium conditions’, Journal of Fish Diseases, 24 (6), pp. 333–340.
Levsen, A. and Jakobsen, P.J. (2002) ‘Selection pressure towards monoxeny in Camallanus cotti’, Parasitology, 124 (5), pp. 589–598.
Moravec, F. (1994) Parasitic Nematodes of Freshwater Fishes of Europe. Prague: Academia.
Wu, S.-G., Xie, M.-Q., Li, A.-X., Zhang, J.-Y. and Fang, Y.-Y. (2007) ‘Population dynamics and maturation cycle of Camallanus cotti (Nematoda: Camallanidae)’, Aquaculture, 266 (1–4), pp. 48–56.
Levamisole: efficacy, pharmacology, and dosing rationale
Yanong, R. (2011) Use of Levamisole in Ornamental Fish. Gainesville, FL: University of Florida IFAS Extension Publication FA091.
Woo, W.-S., Kim, J.-Y., Park, E. and Lee, K. (2023) ‘Exploring the efficacy and safety of levamisole: a review of its broad-spectrum antiparasitic activity’, Frontiers in Pharmacology, 14, 108321.
DrugBank (2023) Levamisole – Pharmacological and Toxicological Data Summary. Available at: https://go.drugbank.com/drugs/DB00848 (Accessed 30 October 2025).
U.S. Drug Enforcement Administration (2022) Drug and Chemical Evaluation: Levamisole. Washington, DC: DEA.
Praziquantel (for comparison with levamisole)
Norbury, L.J., Procopio, D.R., and Terlizzi, M. (2022) ‘Praziquantel use in aquaculture – current status and prospects’, Aquaculture and Fisheries, 7 (4), pp. 367–376.
Mahdy, O.A., El-Shazly, A.M. and Abdel-Aal, A.M. (2024) ‘Evaluation of praziquantel in Nile tilapia (Oreochromis niloticus)’, BMC Veterinary Research, 20 (1), pp. 112–120.
Veterinary Information Network (VIN) (2023) Summary of Praziquantel Application in Fish Medicine. Available at: https://www.vin.com (Accessed 30 October 2025).
Product information and data sheets
eSHa Laboratories (2025) eSHa NDX – Product Data Sheet. Rotterdam: eSHa Laboratories BV.
Fritz Aquatics (2025) Fritz Expel-P – Product Specification and Safety Data Sheet. Mesquite, TX: Fritz Industries.
Cloverleaf Aquatics (2025) Absolute Wormer (Invert Friendly / Aquarium Wormer) – Manufacturer’s Specification. Kent: Cloverleaf Products Ltd.
Harkers Ltd (2025) Harka-Verm – Product Label and Safety Information (8% w/v Levamisole HCl formulation). Bradford: Harkers Ltd.
Authoritative general texts
Anderson, R.C. (2000) Nematode Parasites of Vertebrates: Their Development and Transmission. 2nd edn. Wallingford: CABI Publishing.
Noga, E.J. (2010) Fish Disease: Diagnosis and Treatment. 2nd edn. Ames, IA: Wiley-Blackwell.
Woo, P.T.K. and Bruno, D.W. (2014) Fish Diseases and Disorders, Vol. 3: Viral, Bacterial and Fungal Infections. Wallingford: CABI Publishing.
Contextual References (Hobby and Online Sources)
The following sources are included to illustrate examples of the advice and terminology that have contributed to confusion about Camallanus biology and Levamisole treatment within the aquarium hobby.
They are cited not as evidence supporting this guide, but as representative samples of commonly circulated hobby material where misconceptions have appeared.
Referencing these articles ensures transparency about where outdated claims originate and helps readers understand why clarification is necessary.
Aquarium Co-Op (2022) How to Treat Camallanus Red Worms. Available at: https://www.aquariumcoop.com/blogs/aquarium/camallanus-worms (Accessed 30 October 2025).
Aquarium Co-Op Forum (2022) Camallanus Worms Will NOT STOP!!! Available at: https://forum.aquariumcoop.com/topic/22295-camallanus-worms-will-not-stop/ (Accessed 30 October 2025).
AquariumScience.org (2024) 10.12 Nematodes in Aquarium Fish: Camallanus. Available at: https://aquariumscience.org/index.php/10-12-nematodes/ (Accessed 30 October 2025). Tropical Fish Keeping UK (2022) Camallanus Worms. Available at: https://tropicalfishkeeping.co.uk/camallanus-worms (Accessed 30 October 2025).
Aquariums West (2019) What Are Camallanus Worms? Available at: https://aquariumswest.com/blogs/helpful-articles/what-are-camallanus-worms (Accessed 30 October 2025).
FishKeeping.co.uk Forum (2016) Camallanus Worms Outbreak and How to Get Rid of Them. Available at: https://www.fishkeeping.co.uk/modules/newbb/viewtopic.php?post_id=343867 (Accessed 30 October 2025).
Jen and Nora (2024) Fighting Internal Fish Parasites: Capillaria and Camallanus Worms. Medium. Available at: https://medium.com/@fishkeepandchill/fighting-internal-fish-parasites-capillaria-and-camallanus-worms-eb5d3bb288f8 (Accessed 30 October 2025).
Notes
All URLs and access dates reflect current availability (as of 30 October 2025).
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