Worming Fish: Should You Dose the Food or the Water?
- Macauley Sykes
- Oct 31
- 7 min read
Updated: 2 days ago
Few topics spark more debate among aquarists than how to administer deworming treatments. Some insist that adding medication to the water is the only reliable approach.
Others argue that medicated food delivers a cleaner, more targeted cure. Both camps can point to real-world successes, yet the truth is more nuanced than either side likes to admit.
The question matters more than it might seem. Worms can devastate fish health if left untreated, and choosing the wrong delivery method can make even the right medication fail.
Aquarists often find themselves repeating courses or switching products, not realising that the issue lies not with the drug itself but with how it was given.
This guide looks at what the evidence actually shows. It explains when dosing treatments into the water offers stronger, more consistent results, and when feeding medicated food remains the better option.
The goal is simple: to replace speculation with understanding and help aquarists choose the most effective and reliable method for their fish.
The Main Medications
Four main worming agents are used in ornamental aquaria:
Levamisole Hydrochloride
Praziquantel
Fenbendazole
Flubendazole
They do not all behave in the same way.
Understanding the differences is the key to choosing whether to dose the food or the water.
Levamisole Hydrochloride (HCL)
Levamisole is the first line of defence against internal nematodes such as Camallanus and Capillaria. It is specifically designed to be added directly to the water, where it dissolves easily and is absorbed through the gills and skin. Once inside the bloodstream, it paralyses adult worms so that they can be expelled naturally.
All aquarium formulations use the hydrochloride (HCl) salt rather than the base form. This distinction is important. The hydrochloride version dissolves completely and remains stable throughout treatment, giving consistent and reliable results.
When used at the correct rate, products such as eSHa NDX and Fritz Expel-P provide dependable systemic coverage through water dosing and clear most nematode infections safely and effectively.
Praziquantel
Praziquantel is used to treat tapeworms and flukes rather than roundworms.
It is one of the safest and most dependable medications that can be added directly to the water.
Although the raw compound is hydrophobic and practically insoluble in water, modern aquarium formulations have solved this by using solubilising or suspension agents that keep the drug evenly dispersed. This allows it to be absorbed through the gills and skin, producing consistent concentrations within the bloodstream without the need for feeding.
Research on carp, goldfish, tilapia, and seabream shows that immersion treatments can achieve the same, or even higher, blood levels than oral dosing. In practice, this means that water dosing is both effective and predictable.
Feeding Praziquantel can still be useful in controlled hatchery systems where feed weights and biomass are measured precisely, but for home aquaria, dosing the water remains the simpler and safer option.
Benzimidazoles (Fenbendazole and Flubendazole)
Benzimidazoles behave differently from Levamisole Hydrochloride and formulated Praziquantel. These compounds are poorly soluble in water and act primarily within the intestinal tract. For these drugs to work most effectively, they should be administered orally. Incorporating them into a gel or oil-based food helps the medication bind to the feed and reach the intestine intact. Liquid formulations such as NT Labs Flukasol can be used temporarily in the water when fish refuse food, but their true strength lies in oral delivery once feeding resumes.
In practical fishkeeping, the use of benzimidazoles is rarely necessary.
For the majority of worm infestations, dosing the water with Levamisole Hydrochloride or Praziquantel remains the faster, simpler, and safer approach.
In Brief: Choosing the Right Wormer
Levamisole Hydrochloride (HCl)
Treats roundworms such as Camallanus and Capillaria.
Fully water-soluble; absorbed through gills and skin.
Dose directly into the water for safe, tank-wide coverage.
Reliable first-line treatment for internal nematodes.
Praziquantel
Targets tapeworms and flukes.
Modern formulations stay evenly dispersed in water.
Absorbed systemically through gills and skin.
Water dosing is effective, predictable, and gentle on fish and filters.
Benzimidazoles (Fenbendazole & Flubendazole)
Poorly soluble; act only inside the gut.
It must be delivered with food to work properly.
For most cases, water dosing with Levamisole or Praziquantel is faster, simpler, and safer.
Levamisole HCL and Praziquantel-Based 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).
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)
Fritz Expel-P (US / Canada)
A Levamisole Hydrochloride-based treatment formulated for ornamental freshwater fish, Fritz Expel-P provides rapid systemic control of internal nematodes such as Camallanus and Capillaria.
Its water-soluble format allows for even dispersion throughout the aquarium, ensuring every fish receives treatment regardless of appetite.
Favoured in the United States for its ease of use and compatibility with a wide range of species, Expel-P is often regarded as the gold standard for immersion deworming.
Active ingredient: Levamisole Hydrochloride (HCl) – 113 mg per packet (0.113 g)
eSHa gdex (UK / EU)
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.
The drop-based dosing system offers simplicity and accuracy, which is especially helpful in small tanks where precise measurement matters.
Active ingredient: Praziquantel, 66 mg per mL
Hikari PraziPro (US / International)
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.
PraziPro’s gentle, plant- and invertebrate-safe formulation has made it a trusted choice for aquarists and professional breeders across the United States.
It is particularly valued for its ease of dosing, consistent dispersion, and compatibility with established biological filtration.
Active ingredient: Praziquantel - 5.0 mg per mL
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.
Comments