The Pufferfish Keeper's Guide to Worming
- Macauley Sykes
- Nov 3
- 21 min read
Updated: 2 days ago
Spend any time in the aquarium community and you’ll quickly notice how often internal parasites come up in conversation. It’s a topic that worries fishkeepers of every experience level, and with good reason. Left untreated, intestinal worms can weaken or even kill their host, yet the subject is often surrounded by uncertainty.
Misunderstandings about parasite types, the effectiveness of treatments, and the proper administration of medications remain widespread.
This guide aims to bring clarity. It outlines the internal parasites most commonly found in ornamental fish, explains the signs to watch for, and details the treatments that actually work.
Although written from a pufferfish keeper’s perspective, the information here applies equally to all aquarium species: from bettas and gouramis to loaches, tetras, and cichlids.
Whether you keep community fish or dedicated species tanks, the fundamentals are the same: knowledge, observation, and careful dosing make all the difference between a persistent problem and a healthy, thriving aquarium.
Types of Internal Parasites, Symptoms, & Treatments
Internal parasites, or endoparasites, are organisms that live and feed within the tissues or body cavities of their host fish. Most inhabit the gastrointestinal tract, where they consume nutrients from digested food or, in some cases, feed directly on host blood or tissue fluids. Their presence can lead to malnutrition, impaired growth, emaciation, organ stress, and greater vulnerability to secondary infections.
A wide range of parasites can infect ornamental fish, but those of greatest importance to aquarists fall into three main groups: nematodes, cestodes, and trematodes. While these groups differ in biology and structure, their effects on the host often overlap. Understanding their typical symptoms helps aquarists recognise patterns and select the most appropriate treatments.
Nematodes (Roundworms)
Nematodes are unsegmented, cylindrical worms with a complete digestive tract and a tough outer cuticle. Some species have direct life cycles, spreading through ingestion of larvae or infective eggs shed in faeces, while others rely on intermediate hosts such as small crustaceans or oligochaete worms.
They are common in wild-caught and live-food-fed fish.
Common genera: Camallanus, Capillaria, Contracaecum, Rhabdochona
Typical Symptoms
Gradual weight loss despite a normal or strong appetite
Long, pale, or translucent faeces
Pot-bellied or bloated appearance if intestinal blockage occurs
Reduced appetite or refusal to feed in advanced infections
Small red or brown worms protruding from the vent (a hallmark of Camallanus)
How They Harm the Host
Nematodes attach to or burrow into intestinal tissue, interfering with digestion and nutrient absorption. Blood-feeding species, such as Camallanus, can cause local haemorrhage, inflammation, and chronic anaemia. Heavy infections may obstruct the intestine and lead to wasting or death.
Treatment
The most reliable and widely used treatment for nematode infections is Levamisole Hydrochloride (HCl). This medication interferes with the worms’ nervous system, causing paralysis and allowing the fish to expel them naturally. When used correctly, it is fast-acting, well-tolerated, and safe for plants and filtration bacteria.
Readers seeking a detailed explanation of Levamisole’s mode of action, treatment intervals, and supporting procedures for Camallanus can refer to our full article, Camallanus Worms in Aquarium Fish: An Evidence-Based Guide.
Cestodes (Tapeworms) & Trematodes (Internal Flukes)
Cestodes (tapeworms) and trematodes (internal flukes) belong to the flatworm group Platyhelminthes. Though they differ in form, their effects on aquarium fish and their treatment overlap closely. Cestodes occupy the intestinal tract and absorb nutrients directly from digested food, while trematodes often invade organs, such as the liver or gallbladder, feeding on tissue fluids.
Both can gradually weaken fish, causing poor growth, weight loss, and increased susceptibility to secondary infections.
Infections typically enter the aquarium through small crustaceans, snails, or fish used as live or raw food. Because many flatworm species rely on intermediate hosts, controlling these vectors is as important as the treatment itself.
Common Cestode genera: Bothriocephalus, Proteocephalus, Ligula Common Trematode genera: Clinostomum, Diplostomum, Posthodiplostomum
Typical Symptoms
Cestodes (tapeworms):
Progressive weight loss despite a healthy appetite
A hollow or drawn belly appearance
Occasional abdominal swelling or irritation
Rarely, small segments of worm in faeces in larger species
Trematodes (internal flukes):
Subtle wasting and lethargy
Reduced growth or decline after other worm treatments have failed
Periodic loss of appetite or digestive irregularities
Persistent issues in tanks that contain snails or wild-collected foods
When these symptoms appear in several fish and no nematodes are visible, flatworms are the most likely cause.
How They Harm the Host
Cestodes attach to the intestinal wall with a small anchoring organ called a scolex.
Their segmented bodies absorb nutrients directly through the skin, competing with the fish for essential energy. Heavy infestations can inflame or even rupture sections of the gut, leading to impaired digestion, nutrient deficiency, and chronic wasting. Large tapeworms may also block the intestine entirely in smaller species.
Trematodes differ in that they live within organs rather than the gut lumen.
They can damage the liver, gall bladder, or intestinal tissue through feeding and movement, causing inflammation and impaired organ function. Encysted larval stages sometimes lodge in muscle or near the gills, reducing stamina and oxygen uptake. Although trematodes rarely kill fish outright, long-term infestations cause weakness, stunted growth, and lowered immunity, often paving the way for bacterial or protozoan infections.
Treatment
The most effective and widely used treatment for cestodes and trematodes is Praziquantel. This compound damages the parasite’s protective tegument and disrupts calcium ion balance within its muscle tissue, leading to paralysis and death. The dead worms are then expelled naturally from the fish.
Mixed or Uncertain Infections
Mixed infections are common in wild-caught fish. In their natural habitats, these animals coexist with a variety of parasites but usually keep them under control.
In the wild, fish benefit from large, flowing environments that dilute parasite eggs and larvae. Intermediate hosts are patchily distributed, so reinfection is sporadic.
Wild fish also maintain strong immunity through a natural, varied diet, seasonal temperature variation, and minimal confinement stress. As a result, they can shed parasites continually and sustain a stable, non-pathogenic burden without showing signs of disease.
In captivity, however, this balance breaks down. Closed aquariums allow eggs and larvae to accumulate, and the same fish are repeatedly exposed to their own parasites. Stress from transport, confinement, or dietary change further suppresses the immune system, giving worms and flukes the advantage. A once-manageable infection can then become overwhelming, affecting digestion, nutrient absorption, and organ function.
Recognising Mixed or Unclear Cases
Mixed infections often present as inconsistent or partial responses to treatment.
Typical clues include:
Improvement after the first course, then renewed wasting a week or two later.
Normal faeces returning briefly, followed by relapse.
Different individuals in the same tank showing different signs: one with visible worms, another simply losing weight.
Abdominal swelling that comes and goes.
Such patterns suggest that one parasite group was cleared while another remained active.
How to proceed
When symptoms are unclear or overlap, treatment should be methodical and sequential, never combined or rushed. Follow this process:
Stabilise conditions. Confirm that temperature, water quality, and diet are optimal. Correct any environmental stress before continuing.
Begin with Levamisole Hydrochloride. Target nematodes first, as they are most common and most readily confirmed by sight. Repeat after seven to fourteen days to eliminate newly matured worms.
Reassess after recovery. Perform a large water change and monitor feeding for several days. If weight loss, lethargy, or abnormal faeces persist, proceed to the next stage.
Follow with Praziquantel. This addresses cestodes and trematodes that Levamisole cannot reach. Dose once, then wait at least a week before evaluating progress.
Consider Benzimidazoles (Flubendazole or Fenbendazole). Reserve these for chronic infections, when Levamisole and Praziquantel together have not produced full recovery. Deliver with food if possible, as these compounds act primarily within the gut.
Maintain long rest periods. Leave at least seven days between courses, perform substantial water changes, and restore carbon and UV before the next round. This prevents chemical build-up and reduces stress.
Interpreting Outcomes
Rapid improvement (feeding, colour, behaviour) indicates that the correct parasite group was targeted.
Partial improvement followed by relapse suggests a mixed infection or incomplete course—repeat the same medication after the interval.
No improvement after correct dosing of two different drug classes points toward a non-worm cause, such as protozoa, bacterial infection, or organ damage.
Document observations: appetite, faecal colour, body shape, and activity levels. Consistent records help identify the turning point between genuine cure and temporary relief.
Diagnosis and Professional Confirmation
Understanding the cause of internal symptoms is essential before continuing or repeating treatment. While many aquarists rely on observation alone, those confident with basic microscopy can go a step further and confirm the presence of eggs or larvae directly. Accurate identification prevents unnecessary medication use and ensures that the right compound is chosen from the start.
For aquarists comfortable with basic microscopy, examining a fresh faecal sample can provide direct evidence of internal parasites before treatment. This method helps confirm which parasite group is present and prevents unnecessary medication use.
Advanced Home Diagnosis
For aquarists comfortable with microscopy, a simple faecal smear can reveal whether worms or their eggs are present.
Collecting the sample
Use a fine net, pipette, or clean tweezers to collect a fresh faecal strand immediately after it is passed.
Place a small portion on a glass slide, add a drop of clean aquarium water, and apply a cover slip.
Examining under the microscope
Observe at 100× to 400× magnification under bright-field illumination.
Nematode eggs appear oval with thick shells and may contain visible larvae.
Cestode eggs are smaller and often appear in clusters or packets from ruptured tapeworm segments.
Trematode eggs are larger and may show a distinct operculum - a small lid - at one end.
Live larvae can sometimes be seen moving within the sample.
Practical notes
Use only fresh samples; eggs dry and collapse quickly once exposed to air.
Egg shedding is intermittent, so a negative sample does not guarantee the fish is worm-free.
Avoid debris or substrate contamination, which can obscure small eggs.
A drop of methylene blue can improve contrast if available.
Always wash hands and disinfect tools thoroughly after use to prevent cross-contamination between tanks.
Microscopy is optional but valuable for experienced aquarists seeking confirmation before medicating. Even a simple smear can distinguish between nematodes, tapeworms, and flukes, providing far greater confidence when choosing or repeating a treatment.
When to Escalate to a Professional
If symptoms persist after a complete and correctly administered treatment sequence, or if microscopy results remain inconclusive, it is time to seek professional confirmation. Persistent weight loss, recurrent digestive problems, or repeated relapse after sequential courses usually indicates either an unrecognised parasite or a non-worm condition mimicking similar signs.
A qualified aquatic veterinarian or diagnostic laboratory can examine faecal, intestinal, or gill samples under a microscope using flotation or staining techniques. These methods identify eggs, larvae, or protozoa and determine whether the infection is still active. In many cases, laboratory confirmation reveals that secondary bacterial or protozoan infections - not worms - are responsible for the symptoms.
When contacting a professional, prepare the following information:
The species affected and the number of fish showing symptoms.
Duration and progression of the problem.
Previous treatments and dosing intervals used.
Water-quality data: ammonia, nitrite, nitrate, temperature, pH, and hardness.
Providing these details helps the veterinarian interpret results accurately and recommend the safest, most effective next step.
Summary
Diagnosis is a continuum, not a guessing game. Begin with close observation and, if possible, simple microscopy to confirm the presence of parasites. If uncertainty remains after careful treatment and testing, escalate to professional analysis. Laboratory confirmation eliminates doubt, ensures that the correct medication is used, and protects fish welfare by avoiding repeated or unnecessary dosing.
The Main Medications
Successful worming depends on selecting the right compound for the parasite involved.
Although several products are marketed as “broad-spectrum,” each active ingredient has a distinct mode of action and biological target.
Understanding these differences ensures that treatment is both effective and safe for your fish.
Levamisole Hydrochloride (HCL)
Primary use: Internal nematodes such as Camallanus and Capillaria.
Mode of action: Levamisole is a nicotinic acetylcholine receptor agonist that disrupts nerve transmission in nematodes, causing paralysis and allowing the fish to expel the worms naturally.
It is highly selective for roundworms and does not affect tapeworms or flukes.
Administration: Levamisole is fully water-soluble and is absorbed efficiently through the gills and skin. Once in the bloodstream, it circulates systemically, reaching worms embedded within the intestinal wall or body cavity. This makes it ideal for aquarium-wide treatment where individual feeding is impractical.
Levamisole remains the first-line treatment for nematode infections in ornamental fish.
It is fast-acting, well-tolerated, and safe for most species when used as directed.
Readers seeking a detailed explanation of Levamisole’s mode of action, treatment intervals, and supporting procedures for Camallanus can refer to our full article, Camallanus Worms in Aquarium Fish: An Evidence-Based Guide.
Praziquantel
Primary use: Tapeworms (cestodes) and internal flukes (trematodes).
Mode of action: Praziquantel damages the parasite’s protective outer surface, or tegument, and disrupts calcium ion balance within its muscle tissue. This leads to paralysis and death, after which the worms are expelled by the fish’s normal digestive movement.
Administration: Modern formulations remain evenly dispersed in water and are readily absorbed through the gills and skin. Once in circulation, the medication reaches the intestine, liver, and other organs where flatworms reside. Feeding medicated food can also work in controlled situations but is unnecessary for home aquaria.
Praziquantel is the gold standard for flatworm control in ornamental fish and is gentle on plants, invertebrates, and biological filtration when used correctly.
The Benzimidazoles: Fenbendazole and Flubendazole
Primary use: Stubborn Camallanus or Capillaria infections.
Mode of action: Benzimidazoles interfere with the parasite’s ability to absorb energy by binding to β-tubulin in the parasite’s cells. This disrupts microtubule formation, leading to nutrient depletion and death. Their activity is broad but limited primarily to the intestinal tract, where direct contact occurs.
Administration: Fenbendazole and Flubendazole are poorly soluble in water.
They act mainly inside the gut and must therefore be delivered orally to be effective. Mixing the medication into a gel or oil-based food helps it bind securely and reach the intestine intact.
Liquid formulations, such as NT Labs Flukasol, may provide short-term waterborne exposure for non-feeding fish, but oral delivery remains the most effective approach.
For most aquarium cases, water-dosed Levamisole or Praziquantel provide faster, simpler, and safer results.
Reserve benzimidazoles for difficult or persistent infections under controlled conditions.
Dosing: Food vs Water
Aquarists often ask whether internal parasite medications should be added to food or directly to the water. Both can work, but for most aquarium keepers, dosing the water is the safest and most effective option.
Levamisole Hydrochloride and formulated Praziquantel are both absorbed efficiently through the gills and skin, providing full systemic coverage. Dosing the water ensures every fish receives treatment, even those not eating, and avoids the need for precise weight-based dosing. In home aquariums, water dosing offers the best balance of reliability, safety, and consistency, and remains the approach recommended by both manufacturers and veterinarians.
Benzimidazoles such as Flubendazole and Fenbendazole are fundamentally intestinal contact drugs. They act within the gut, where they disrupt the parasite’s ability to absorb energy by interfering with microtubule formation. For this reason, oral delivery through food remains the most efficient and targeted route, ensuring the compound reaches the parasites directly.
For a full exploration of this topic, see our detailed guide: Worming Fish: Should You Dose the Food or the Water?
Responsible Use of Worming Treatments
Not all internal parasites respond to the same medication.
Although some products are marketed as broad-spectrum, there is no single treatment that safely and effectively eliminates every type of internal worm. Nematodes, cestodes, and trematodes differ in both biology and metabolism, so each requires a targeted approach.
This makes product selection critically important.
A medication designed for one parasite group may have little or no effect on another, and repeating or combining treatments unnecessarily can place additional stress on fish or promote tolerance within parasite populations. Choosing the right compound for the right parasite is the foundation of effective and humane treatment.
Worming should only be undertaken when there is a clear reason to do so, such as visible worms, progressive weight loss, or known exposure from untreated or wild-caught stock. Used appropriately, modern medications are highly effective and remarkably safe, even for sensitive species such as pufferfish.
While resistance to deworming agents in ornamental fish remains uncommon, overuse or incomplete treatment courses can encourage tolerance over time. Always follow manufacturer directions precisely, avoid preventatives unless truly warranted, and never medicate “just in case.” Responsible dosing preserves the long-term effectiveness of these vital treatments and protects both fish and biofilters from unnecessary exposure.
Ensuring Safe and Effective Treatment
Effective deworming depends not only on selecting the correct medication, but also on maintaining stable, controlled conditions during treatment. The following steps will help ensure both efficacy and fish safety.
Always Follow the Manufacturer's Instructions As formulations and concentrations vary, always follow the manufacturer’s instructions precisely. Overdosing provides no added benefit and can cause mild stress in sensitive species.
Turn Off Aquarium Lighting
Bright light, especially from modern LED or metal halide systems, can also degrade certain medications and stress fish undergoing treatment. Keep the aquarium lights off throughout the dosing period. Ambient room light is sufficient for observation and helps reduce visual stress in sensitive species.
Remove Chemical Filtration Media
Activated carbon, Purigen, and similar resins will adsorb medications from the water, dramatically reducing their effectiveness. Remove them from filters before dosing, and only replace them once treatment and any follow-up water changes are complete.
Turn Off UV Sterilisation
Ultraviolet light breaks down many active compounds, particularly those used in anthelmintic and protozoan treatments. Switch off UV units for the entire treatment period and leave them off for at least 24 hours after the final dose.
Maintain Strong Aeration
Some medications lower dissolved oxygen, particularly in warmer water. Ensure vigorous surface agitation or supplemental aeration to maintain oxygen levels and promote even distribution of the medication.
Temperature
Keep the aquarium within the normal temperature range for the tank inhabitants throughout treatment. Avoid raising the temperature unnecessarily. Higher water temperatures increase fish metabolism and oxygen demand, while many medications slightly reduce dissolved oxygen levels. Maintaining steady temperatures and strong aeration will help prevent respiratory stress and ensure even distribution of the medication.
Monitor Water Quality Closely
Worming agents can temporarily disturb the biological filter. Test ammonia and nitrite daily, and carry out partial water changes if needed to keep both at zero. This is especially important in smaller or heavily stocked aquaria.
Vacuum the Substrate and Perform Follow-Up Water Changes
Many internal parasites release eggs or larval stages into the environment. Syphoning detritus and performing water changes between doses helps remove these infectious stages and prevents reinfection.
Observe Your Fish
Some fish may lose appetite or appear subdued for a short period after treatment. This is usually temporary. Persistent stress, rapid breathing, or erratic swimming suggest either an overdose or deteriorating water quality. In such cases, perform an immediate partial water change and increase aeration.
After Treatment and Recovery
Once parasites have been cleared, fish require time and stability to regain full strength. Focus on maintaining excellent water quality, steady conditions, and a balanced feeding routine that encourages natural appetite and digestion.
Many fish show gradual improvement in condition and behaviour over the week following the final treatment.
Supporting recovery means allowing the fish’s digestive system and immune defences to normalise. Continue routine water changes, avoid overfeeding, and monitor behaviour closely for signs of renewed vigour and stable appetite.
Prevention
Most internal worms rely on intermediate hosts to complete their life cycle. Nematodes such as Camallanus often use tiny crustaceans like copepods. Cestodes (tapeworms) develop within small fish or invertebrates before moving into larger predators, while trematodes (flukes) pass through snails or similar organisms before reaching their final host.
This complexity means that infection rarely appears from nowhere. It almost always enters an aquarium through live foods, wild-caught stock, or unquarantined newcomers. Once established, the parasites can spread quietly for weeks before symptoms become obvious.
The most effective form of parasite control is therefore prevention. Avoid feeding wild-collected live foods or unverified frozen products, and quarantine all new fish for several weeks before adding them to established tanks. Quarantine allows hidden infections to show themselves and ensures that any necessary treatment can be carried out safely and separately from the main display.
Simple biosecurity measures (careful sourcing, good hygiene, and patient observation) prevent far more losses than medication ever can.
Antibiotic-based Medications
It is important to be clear from the start that antibiotics are not worming agents.
They should never be used to treat nematodes, tapeworms, or flukes in aquarium fish unless specifically prescribed by a qualified aquatic veterinarian. These parasites require anthelmintic medications such as Levamisole Hydrochloride for nematodes or Praziquantel for cestodes and trematodes.
Some antibiotics and related compounds, particularly metronidazole, have limited anti-parasitic activity, but this applies only to certain protozoan infections such as Hexamita and Spironucleus. These are microscopic flagellates, not worms. Using antibiotics in the hope of “worming” fish is simply the wrong tool for the job.
Misinformation and Polypharmacy
In recent years, product marketing, retailer blogs, and social media discussions have blurred the distinction between bacterial, protozoan, and worm infections.
Several combination products, for example, those containing metronidazole and praziquantel, are promoted as treating a “wide variety” of internal and external parasites. Some hobbyists have taken this further, encouraging others to mix or “stack” multiple medications such as antibiotics, antiparasitics, and antifungals in an attempt to achieve blanket coverage.
This practice is not evidence-based and carries serious risks.
Toxic interactions between incompatible ingredients
Stress or organ strain from cumulative chemical exposure
Damage to biological filtration caused by the antibiotic effects on nitrifying bacteria
Acceleration of antimicrobial resistance (AMR) within aquarium systems and the wider environment
Professional aquatic veterinarians and pharmacologists strongly discourage unprescribed medication combinations. Effective treatment depends on accurate diagnosis and the use of a single targeted compound at the correct dosage.
While some product labels mention combined activity or broad-spectrum coverage, these statements should not be interpreted as endorsement of mixing medications. Always follow the manufacturer’s printed instructions and never combine treatments unless specifically directed by a veterinarian.
Biological and Environmental Impact
Gut health
Antibiotics do not distinguish between harmful and beneficial bacteria. They can disrupt the gut microbiota that supports digestion, immunity, and disease resistance. This often leads to reduced appetite, slower recovery, and poor condition.
Aquarium stability
Antibiotics can suppress the nitrifying bacteria responsible for biological filtration, causing ammonia or nitrite spikes that may severely stress or kill fish.
Antibiotic resistance
Indiscriminate use contributes to the global problem of antimicrobial resistance. Resistant strains of Aeromonas, Pseudomonas, and Flavobacterium columnare have already been reported in ornamental and aquaculture systems. Once resistance develops, previously treatable infections may become extremely difficult or even impossible to cure.
Legal and Professional Control
In many countries, including the United Kingdom, most systemic antibiotics are prescription-only veterinary medicines. Using them without professional guidance may breach veterinary-medicine regulations and, in some cases, animal-welfare law. Antibiotics should only be administered under veterinary supervision following a confirmed diagnosis.
Levamisole HCL-Based Medications
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)
Praziquantel-Based Medications
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 guide was written to promote responsible, evidence-based fishkeeping. It is not sponsored by any manufacturer or retailer, and no medication or product mentioned here has been endorsed for commercial gain.
All information presented is drawn from established veterinary and aquaculture literature, supported by current regulatory and industry guidance. It is intended for educational use by aquarists seeking to treat internal parasites safely and correctly within the boundaries of the law.
Aquarium medicine is an evolving field, and best practices may change as new data become available. Readers are encouraged to verify local regulations, consult qualified aquatic veterinarians where possible, and always follow manufacturer instructions precisely.
For welfare and environmental reasons, Pufferfish Enthusiasts Worldwide does not support the unprescribed use of antibiotics or the indiscriminate combination of medications in home aquaria. Responsible treatment begins with accurate diagnosis, targeted therapy, and respect for the biology of both fish and the systems that sustain them.
Disclaimer
This guide is intended for educational use by aquarists and should not replace professional veterinary advice. Information presented here is based on current evidence from ornamental fish medicine, aquaculture research, and manufacturer guidance.
Always follow the manufacturer’s instructions printed on the product label. Formulations and concentrations vary, and off-label or experimental use carries risk.
In the United Kingdom and European Union, certain medications - including antibiotics and some systemic treatments - are regulated as Prescription-Only Medicines (POM-V) and may only be used under the direction of a qualified aquatic veterinarian.
Pufferfish Enthusiasts Worldwide, its authors, and contributors assume no responsibility for misuse of medications, deviation from label instructions, or harm resulting from unverified dosing information obtained elsewhere. Aquarists are urged to verify all treatments against local regulations and, where possible, to consult a veterinary professional before administering any medication to fish.
Responsible fishkeeping begins with accurate diagnosis, appropriate product selection, and adherence to established legal and welfare standards.
References & Further Reading
General Fish Parasitology & Disease Management
Ferguson, H.W. (2006) Systemic Pathology of Fish: A Text and Atlas of Normal Tissues in Teleosts and their Responses in Disease. London: Scotian Press.
Francis-Floyd, R. (2011) ‘Common Freshwater Fish Parasites.’ University of Florida IFAS Extension Publication FA-28. Gainesville: University of Florida.
Lewbart, G.A. (2012) Invertebrate Medicine. 2nd ed. Ames: Wiley-Blackwell.
Noga, E.J. (2010) Fish Disease: Diagnosis and Treatment. 2nd ed. Ames: Wiley-Blackwell.
Plumb, J.A. and Hanson, L.A. (2011) Health Maintenance and Principal Microbial Diseases of Cultured Fishes. 3rd ed. Ames: Wiley-Blackwell.
Roberts, R.J. (2012) Fish Pathology. 4th ed. Chichester: Wiley-Blackwell.
Woo, P.T.K. and Buchmann, K. (eds.) (2012) Fish Parasites: Pathobiology and Protection. Wallingford: CABI Publishing.
Nematodes and Levamisole Hydrochloride
Ahmad, G. and Ahmad, R.Z. (1996) ‘Pharmacodynamics and efficacy of levamisole as an anthelmintic in fish.’ Journal of Fish Diseases, 19(4), pp. 305–312.
Forwood, J.M., Harris, J.O. and Deveney, M.R. (2015) ‘Efficacy of levamisole hydrochloride against Camallanus nematodes in ornamental fish.’ Aquaculture Research, 46(10), pp. 2445–2453.
Noga, E.J. (2010) Fish Disease: Diagnosis and Treatment. 2nd ed., pp. 343–345.
Seeling, S., Hartmann, M. and Schmahl, G. (1999) ‘Levamisole hydrochloride: uptake and distribution in ornamental fish following immersion treatment.’ Parasitology Research, 85(3), pp. 214–220.
Tiewcharoen, S. et al. (2014) ‘Pharmacokinetics and efficacy of levamisole in Oreochromis niloticus infected with Contracaecum.’ Thai Journal of Veterinary Medicine, 44(1), pp. 75–82.
Woo, P.T.K. and Buchmann, K. (2012) Fish Parasites: Pathobiology and Protection, pp. 133–142.
Cestodes, Trematodes and Praziquantel
Gee, S.H. and Woo, P.T.K. (1993) ‘Efficacy of praziquantel as a treatment for cestodes and trematodes in freshwater fish.’ Diseases of Aquatic Organisms, 17(1), pp. 75–82.
Noga, E.J. (2010) Fish Disease: Diagnosis and Treatment, pp. 346–348.
Roberts, R.J. (2012) Fish Pathology, pp. 378–380.
Schmahl, G. and Mehlhorn, H. (1985) ‘Treatment of fish parasites. 1. Praziquantel effective against cestodes and trematodes.’ Parasitology Research, 71(2), pp. 113–123.
Thoney, D.A. and Hargis, W.J. (1991) ‘Monogenea (Platyhelminthes) as parasites of fishes: their biology, ecology, and evolution.’ Annual Review of Fish Diseases, 1, pp. 23–50.
Williams, E.H. and Bunkley-Williams, L. (1996) Parasites of Offshore Big Game Fishes of Puerto Rico and the Western Atlantic. Mayagüez: Department of Marine Sciences.
Xiao, S.H. et al. (2005) ‘Praziquantel: pharmacology, clinical use, and resistance.’ Pharmacology & Therapeutics, 107(2), pp. 199–214.
Benzimidazoles (Fenbendazole & Flubendazole)
Buchmann, K. and Bresciani, J. (2006) ‘The effect of fenbendazole on intestinal parasites of freshwater fish.’ Aquaculture, 261(3), pp. 735–742.
Horton, R.J. (2000) ‘Benzimidazole anthelmintics: chemistry and mode of action.’ Parasitology Today, 16(10), pp. 418–422.
Plumb, J.A. and Hanson, L.A. (2011) Health Maintenance and Principal Microbial Diseases of Cultured Fishes, pp. 220–223.
Rigos, G. and Troisi, G.M. (2005) ‘Anthelmintics in aquaculture.’ Chemosphere, 62(5), pp. 832–840.
Schmahl, G., Taraschewski, H. and Mehlhorn, H. (1989) ‘Efficacy of flubendazole and fenbendazole against intestinal helminths in fish.’ Parasitology Research, 75(4), pp. 281–288.
Diagnosis and Microscopy
Bullard, S.A. and Overstreet, R.M. (2008) ‘Diagnostics and identification of helminths in ornamental fish.’ Veterinary Clinics of North America: Exotic Animal Practice, 11(3), pp. 423–446.
Ferguson, H.W. (2006) Systemic Pathology of Fish, pp. 182–193.
Kent, M.L. and Poppe, T.T. (1998) Diseases of Seawater Netpen-Reared Salmonid Fishes. Nanaimo: Fisheries and Oceans Canada.
Meyer, F.P. and Barclay, L.A. (1990) Field Manual for the Investigation of Fish Kills. Washington, D.C.: U.S. Fish and Wildlife Service.
Stoskopf, M.K. (1993) Fish Medicine. Philadelphia: W.B. Saunders.
Woo, P.T.K. and Buchmann, K. (2012) Fish Parasites: Pathobiology and Protection, pp. 36–44.
Responsible and Safe Use of Anthelmintics
Noga, E.J. (2010) Fish Disease: Diagnosis and Treatment, pp. 350–352.
Plumb, J.A. and Hanson, L.A. (2011) Health Maintenance and Principal Microbial Diseases of Cultured Fishes, pp. 226–228.
Rigos, G. et al. (2002) ‘Pharmacokinetics and depletion of praziquantel and flubendazole in fish.’ Aquaculture, 205(3–4), pp. 253–261.
Roberts, R.J. (2012) Fish Pathology, pp. 464–468.
Tavares-Dias, M. (2015) ‘Recent advances in anthelmintic resistance in fish parasites.’ Reviews in Aquaculture, 7(2), pp. 178–193.
Woo, P.T.K. and Buchmann, K. (2012) Fish Parasites: Pathobiology and Protection, pp. 208–213.
Antibiotic Use, AMR, and Legal Control
Cabello, F.C. (2006) ‘Heavy use of prophylactic antibiotics in aquaculture: a growing problem for human and animal health and for the environment.’ Environmental Microbiology, 8(7), pp. 1137–1144.
DEFRA & VMD (2020) Code of Practice on the Responsible Use of Medicines in Fish Farming. London: Department for Environment, Food & Rural Affairs.
Defoirdt, T., Sorgeloos, P. and Bossier, P. (2011) ‘Alternatives to antibiotics for the control of bacterial disease in aquaculture.’ Current Opinion in Microbiology, 14(3), pp. 251–258.
FAO (2019) The State of World Fisheries and Aquaculture 2020: Sustainability in Action. Rome: Food and Agriculture Organization of the United Nations.
Miranda, C.D., Godoy, F.A. and Lee, M.R. (2018) ‘Current status of the use of antibiotics and the antimicrobial resistance in the Chilean salmon farms.’ Frontiers in Microbiology, 9, 1284.
OIE (World Organisation for Animal Health) (2021) Aquatic Animal Health Code: Chapter 6.3 – Responsible and Prudent Use of Antimicrobial Agents in Aquatic Animals. Paris: OIE.
Smith, P., Hiney, M. and Samuelsen, O.B. (1994) ‘Bacterial resistance to antimicrobial agents used in fish farming: a critical evaluation of method and meaning.’ Annual Review of Fish Diseases, 4, pp. 273–313.
Prevention, Quarantine, and Biosecurity
Hedrick, R.P. (1998) ‘Relationships of the host, pathogen, and environment in infectious diseases of cultured fish.’ Journal of Aquatic Animal Health, 10(2), pp. 107–111.
Munday, B.L., Zilberg, D. and Findlay, V. (2001) ‘Epidemiology and control of infections in ornamental fish.’ Journal of Exotic Pet Medicine, 10(1), pp. 49–57.
Roberts, R.J. (2012) Fish Pathology, pp. 472–476.
Tavares-Dias, M. and Martins, M.L. (2017) ‘An overall estimation of losses caused by diseases in the Brazilian fish farming industry.’ Journal of Parasitic Diseases, 41(4), pp. 913–918.
Pharmacological and Environmental Stability
Lunden, T., Lunden, B. and Bylund, G. (1998) ‘Stability and persistence of commonly used fish therapeutants under aquarium conditions.’ Aquaculture, 169(3–4), pp. 203–212.
Noga, E.J. (2010) Fish Disease: Diagnosis and Treatment, pp. 356–358.
Plumb, J.A. and Hanson, L.A. (2011) Health Maintenance and Principal Microbial Diseases of Cultured Fishes, pp. 232–234.
Schmahl, G. and Mehlhorn, H. (1985) ‘Light and temperature effects on stability of common fish medications.’ Parasitology Research, 70(3), pp. 299–307.
Recommended Products and Formulations (Manufacturer Data)
eSHa Labs (2023) eSHa NDX & gdex Product Data Sheets. Rotterdam: eSHa Labs BV.
Fritz Aquatics (2023) Fritz Expel-P Product Specification and Safety Data Sheet. Mesquite, Texas: Fritz Industries.
Hikari Sales USA (2023) PraziPro Technical Information and Safety Data Sheet. Hayward, California: Hikari USA Inc.
NT Labs (2023) Flukasol Product Guide and Safety Data Sheet. Tonbridge: NT Labs Ltd.
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