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Worming Fish: Should You Dose the Food or the Water?

Updated: Jan 28

Internal parasitic worms are a common concern in ornamental fishkeeping and are frequently managed using generalised advice that does not distinguish between different medications, formulations, or parasites.

The central issue is not whether dosing food or water is “better” in general, but how a specific drug is absorbed and reaches its target parasite.


Once this is understood, treatment decisions become clearer and more consistent.


This article reviews the most commonly used worming agents in ornamental fishkeeping, outlines what is supported by peer-reviewed literature and commercial product use, and explains why water dosing remains the most practical first-line approach for most aquarists.

Routes of medication uptake in fish


Fish may be exposed to antiparasitic medications via two primary routes:

  • Immersion exposure, primarily across the gills and skin

  • Oral exposure, following ingestion and gastrointestinal absorption

Both routes can result in systemic exposure. Their effectiveness depends on the drug’s chemical properties, formulation, exposure duration, species differences, and the biology of the target parasite.


There is no single route that is universally superior for all medications.

The most common worming medications


Almost all aquarium worm treatments use one of these four ingredients:


  • Levamisole hydrochloride

  • Praziquantel

  • Fenbendazole

  • Flubendazole


Each behaves differently and must be considered individually.

  • Levamisole Hydrochloride (HCL)


    What it treats: Roundworms/nemotodes

    Best method: Water (immersion) dosing


    Levamisole is the first line of defence against internal nematodes such as Camallanus and Capillaria. Levamisole hydrochloride is readily water-soluble and is effectively absorbed during immersion treatments. It acts systemically by disrupting neuromuscular function in susceptible nematodes, leading to paralysis and expulsion.

    Immersion dosing is widely used in aquaculture and ornamental contexts and is well-suited to community aquariums. Oral administration offers no clear advantage in most ornamental scenarios and introduces additional variables related to feeding behaviour and dose control.

    Levamisole does not reliably affect eggs or early developmental stages in many nematodes. As a result, repeat dosing at intervals aligned with parasite life cycles is often required, alongside removal of expelled parasites from the system.

    The stability of levamisole in aqueous solutions is influenced by environmental conditions. When used according to product directions and typical treatment windows, it is considered appropriate for immersion use.

    Click here to jump to Levamisole HCL-based medications

  • Praziquantel


    What it treats: Cestodes (tapeworms) and trematodes (internal flukes)

    Best method: Water (immersion) dosing

    Praziquantel has low intrinsic water solubility, but commercially available aquarium formulations include solubilising or dispersing agents that allow effective distribution in water when used as directed.

    Immersion treatment with praziquantel is well established in aquaculture and veterinary medicine. Experimental and applied studies demonstrate that bath exposure can produce meaningful systemic uptake and effective parasite control when appropriate concentrations and exposure durations are used.

    Commercial liquid formulations are designed for water dosing and rely on proper dispersion rather than complete molecular dissolution. Manufacturer instructions typically emphasise thorough mixing prior to dosing.

    For ornamental fishkeeping, water dosing praziquantel is generally the most practical and reliable approach, particularly where fish are stressed, anorexic, or difficult to feed individually.

    Praziquantel is not effective against nematodes and should not be used as a general treatment in the absence of appropriate diagnosis.

    Click here to jump to Praziquantel-based medications


  • Fenbendazole and Flubendazole

    What they treat: Nematodes and some flukes

    Routes: Oral or immersion, depending on formulation and exposure

    Fenbendazole and flubendazole are benzimidazole anthelmintics with poor inherent water solubility. This characteristic has led to the widespread assumption that they must be administered orally.

    Peer-reviewed studies demonstrate that immersion exposure to fenbendazole can be effective against certain parasites when exposure duration and formulation allow sufficient bioavailability. Comparative studies have reported high efficacy for bath treatments under defined experimental conditions.

    Commercial flubendazole products are commonly formulated as suspensions intended for water dosing, particularly in pond and large-volume systems. These products are labelled for immersion use and rely on prolonged exposure rather than rapid dissolution.

    Oral administration remains an effective approach for intestinal parasites when ingestion can be reliably controlled. However, the available literature does not support the assertion that benzimidazoles must be ingested to be effective.


    The determining factors are formulation, exposure duration, and target parasite, rather than route alone

Medication,

Parasites treated

Dosing method

Levamisole hydrochloride

Nematodes such as Camallanus and Capillaria

Dose the water

Praziquantel

Tapeworms and flukes

Dose the water

Fenbendazole

Intestinal nematodes and some cestodes

Often oral, immersion possible

Flubendazole

Intestinal nematodes and some cestodes

Immersion, when properly formulated

Delivery method should always follow product formulation and labelled instructions, not assumptions based solely on drug class.

Why water dosing remains the best default for most aquarists


Oral dosing can be effective under controlled conditions, but it introduces practical challenges that are difficult to manage consistently in ornamental aquaria:

  • Uneven food intake and dominance hierarchies

  • Reduced appetite in sick fish

  • Difficulty controlling individual doses

  • Risk of subtherapeutic exposure

Water dosing avoids these issues by ensuring uniform exposure across all fish in the system. When the medication is suitable for immersion and used according to directions, this approach is more repeatable, less error-prone, and more accessible to beginners.


For these reasons, water dosing remains the recommended first-line approach in most ornamental fish scenarios, while acknowledging that alternative routes may be appropriate in specific, controlled circumstances.


This is a practical recommendation, not a pharmacological absolute.

Water Dosing vs Food Dosing

Aspect

Water dosing

Food dosing

Ease for beginners

Very high.

Measure the tank volume and follow the manufacturer's instructions.

Lower.

Requires accurate food preparation and consistent feeding.

Reliance on appetite

None.

The fish do not need to eat any medication to receive the correct dosage.

High.

Ineffective if the fish are off food or are feeding unevenly (very common for sick fish).

Dose consistency

High.

All fish are exposed to the same dosage simultaneously.

Variable.

Requires weighing and complex calculations. Dominant fish may receive more, and weaker fish may receive less.

Best-suited medications

Levamisole hydrochloride, praziquantel

Fenbendazole, flubendazole

Common failure mode

Incorrect volume calculation or missed repeat dose

Underdosing due to poor or uneven feeding

Beginner risk profile

Lower, when the correct drug is chosen

Higher, even when the correct drug is chosen

Clarifying a common misconception: “fish don’t drink”


It is sometimes claimed that medications added to aquarium water cannot affect internal parasites because fish “do not drink.” While freshwater fish do not drink water in the same way as terrestrial animals, this does not mean they are isolated from substances in their environment.


Many compounds are absorbed across the gills and other body surfaces during immersion exposure, which is why bath treatments are widely used in aquatic veterinary medicine.


The extent of absorption varies by drug, species, and conditions, and immersion does not guarantee therapeutic blood levels in every case, but uptake is not negligible and does not require drinking to occur.

Suitable Medications


Here are some of our member favourites, all of which have been carefully reviewed by Pufferfish Enthusiasts Worldwide.


Where manufacturers’ dosage instructions are followed, we have verified that the working concentrations fall safely within accepted ranges for ornamental freshwater fish (including pufferfish).

Levamisole HCL-Based Medications



eSHa NDX (UK / EU)


Active ingredient: Levamisole Hydrochloride (HCl) – 5.4 % w/v (54 mg per mL)


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.


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.


Fritz Expel-P (US / Canada)


Active ingredient: Levamisole Hydrochloride (HCl) – 113 mg per packet (0.113 g)


A Levamisole Hydrochloride-based treatment formulated for ornamental freshwater fish, Fritz Expel-P provides rapid systemic control of internal nematodes.


Favoured in the United States for its ease of use and compatibility with a wide range of species, Expel-P is often regarded as a gold standard for immersion deworming.

Praziquantel-Based Medications



Hikari PraziPro (US / International)


Active ingredient: Praziquantel - 5.0 mg per mL


A refined liquid formulation developed specifically for ornamental freshwater and marine fish, Hikari PraziPro is designed for the control of tapeworms, flatworms, and flukes.


Its fully solubilised Praziquantel base ensures even distribution throughout the aquarium, allowing systemic absorption through the gills and skin.


eSHa gdex (UK / EU)


Active ingredient: Praziquantel, 66 mg per mL


A specialised anti-helminthic formulated for ornamental freshwater fish, eSHa gdex is designed for the control of tapeworms, flatworms, and flukes.


Its praziquantel suspension disperses evenly in the water, supporting reliable uptake and consistent results when used as directed.

Final remarks


There is no single correct answer to whether fish should be dosed via food or water.


The correct approach depends on:


  • the pharmacology of the drug

  • the formulation used

  • the biology of the target parasite

  • the practical realities of the system


For most aquarists, most of the time, properly formulated immersion treatments provide the safest, simplest, and most consistent solution, particularly at beginner and intermediate levels.


Scientific accuracy and practical husbandry are not in conflict. They reinforce each other when applied correctly.

Author’s Note


This article was written for Pufferfish Enthusiasts Worldwide (PEW) as part of our educational series on responsible aquarium medicine.


All products mentioned refer to aquarium-formulated treatments intended for ornamental fish. Always follow manufacturer instructions and consult a qualified aquatic veterinarian when treating valuable or sensitive species.

Disclaimer


The information provided in this article is for educational purposes only. It does not constitute veterinary advice and should not be used to diagnose or prescribe treatment for any animal.


All medications referenced refer to aquarium-formulated products intended for ornamental fish. Always follow manufacturer instructions, observe local regulations, and consult a qualified aquatic veterinarian if in doubt or when treating valuable livestock.


Pufferfish Enthusiasts Worldwide promotes responsible, evidence-based care and does not encourage the use of raw or prescription-only pharmaceuticals in home aquaria.


References & Further Reading


Pharmacology and Mechanism


  • Anderson, R. C. (2000) Nematode Parasites of Vertebrates: Their Development and Transmission. 2nd edn. Wallingford: CABI Publishing.

  • Reinemeyer, C. R. & Craig, T. M. (2021) Anthelmintics in Veterinary Parasitology. 5th edn. Wiley-Blackwell.

  • Thompson, K. G. et al. (2018) ‘Comparative solubility and dissolution characteristics of benzimidazole anthelmintics’, Veterinary Parasitology, 259, pp. 32–41.

  • Woo, P. T. K. & Bruno, D. W. (2019) Fish Diseases and Disorders, Volume 2: Non-infectious Disorders. 3rd edn. Wallingford: CABI Publishing.


Formulation and Solubility


  • European Directorate for the Quality of Medicines (2023) Levamisole Hydrochloride Monograph, European Pharmacopoeia 11. Strasbourg: EDQM.

  • Jones, P. & Kearn, G. (2022) ‘Praziquantel in aquaculture: a review of pharmacology, formulation and field use’, Reviews in Aquaculture, 14(4), pp. 1903–1920.

  • OECD (2023) Fenbendazole: Solubility and Environmental Fate Summary. Paris: OECD.

  • US FDA (2022) Fenbendazole and Flubendazole Chemical Safety Evaluation. Silver Spring, MD: Center for Veterinary Medicine.

  • Thoney, D. A. & Hargis, W. J. (2023) ‘Monogenea and Digenea of marine fishes and their control’, Annual Review of Fish Diseases, 3, pp. 233–261.


Fish Pharmacokinetics and Efficacy Studies


  • Choi, B. et al. (2024) ‘Efficacy and adverse effects of levamisole bath against Microcotyle sebastis in Korean rockfish (Sebastes schlegelii)’, Aquaculture Reports, 34, 101961.

  • García-Martínez, A. et al. (2024) ‘Praziquantel pharmacokinetics and tissue distribution after immersion exposure in gilthead seabream (Sparus aurata)’, Aquaculture, 586, 740056.

  • Hakalahti, T., Bandilla, M. & Valtonen, E. T. (2004) ‘Efficacy of bath treatments with levamisole hydrochloride against Camallanus lacustris in fish hosts’, Diseases of Aquatic Organisms, 59(3), pp. 211–216.

  • Velisek, J. et al. (2012) ‘Acute toxicity of praziquantel to juvenile common carp (Cyprinus carpio)’, Aquaculture International, 20(4), pp. 665–672.

  • Wagner, E. J. et al. (2008) ‘Evaluation of levamisole hydrochloride bath treatments for control of internal nematodes in rainbow trout’, North American Journal of Aquaculture, 70(3), pp. 330–336.

  • Yoshinaga, T. & Nakazoe, J. (1993) ‘Praziquantel treatment of the monogenean Benedenia seriolae in cultured yellowtail’, Fish Pathology, 28(4), pp. 189–193.


Product and Manufacturer Data


  • Fish Treatment Ltd. (2024) Fluke-Solve Product Data Sheet: Rapidly Dissolving Praziquantel Formulation for Fish. United Kingdom: Fish Treatment Ltd.

  • Hikari USA Inc. (2024) PraziPro Product Safety Data Sheet. Franklin, TN: Hikari USA Inc.

  • NT Labs Ltd. (2024) Flukasol Product Specification Sheet. Kent: NT Labs Ltd.


General References and Context


  • Alvarez-Pellitero, P. & Sitjà-Bobadilla, A. (2020) ‘Parasite infections in cultured marine fish: lessons learned and future directions’, Aquaculture, 519, 734915.

  • Buchmann, K. & Bresciani, J. (2021) ‘Monogenea (Phylum Platyhelminthes): Solved and unsolved problems’, Fish & Shellfish Immunology, 115, pp. 72–83.

  • Cribb, T. H. et al. (2002) ‘The life cycles of fish trematodes and their relevance to aquaculture’, International Journal for Parasitology, 32(3), pp. 321–340.

  • FAO (2021) Responsible Use of Antiparasitic Drugs in Aquaculture. Rome: Food and Agriculture Organization of the United Nations.

  • Rigos, G. & Troisi, G. M. (2005) ‘Antiparasitic drugs in Mediterranean aquaculture: a review’, Aquaculture Research, 36(13), pp. 1262–1277.

  • Roberts, R. J. (2012) Fish Pathology. 4th edn. Oxford: Wiley-Blackwell.

  • Stoskopf, M. K. (2010) Fish Medicine. Philadelphia: Saunders Elsevier.

  • Kearn, G. C. (2011) Parasitism and the Platyhelminthes. Oxford: Oxford University Press.

 
 
 

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