Fenbendazole (dewormer)

(DO NOT dose or food soak fenbendazole in a reef environment.)

What It Treats: Flukes (Monogeneans) and intestinal worms. Fenbendazole is particularly useful to treat flukes & intestinal worms which seem resistant to praziquantel.

How To Treat: Fenbendazole is a drug often prescribed by veterinarians to treat intestinal worms in dogs & cats (sometimes sold as Panacur.) It is also effective to use on fish, and can be purchased online here & here.

Noga discusses using fenbendazole to treat monogeneans (flukes) in his book Fish Disease: Diagnosis and Treatment 2nd Edition. He recommends a 12 hour bath treatment at 25 mg/L (95 mg/gallon). Heavily aerate + temp control the bath water. Fully dissolve the medication before adding fish. Fenbendazole is not easily water soluble, so I advise using either ethyl alcohol or preferably DMSO to aid with dissolving it: How To – Use DMSO to dissolve medications

I advise transferring the fish into a new QT following each bath treatment (to prevent reinfection). You will also need to administer a second bath + transfer about a week later in order to eliminate any hatchlings. It is best to use this treatment calendar to determine when is the best time to do the second bath: BeNeZe – Cawthron 2022

When using the treatment calendar (screenshot below): Be sure “Parasite Selection” is checked for all 3, and below that move the water temperature slider for your treatment tank water temp in Celsius. (Fahrenheit to Celsius conversion here.)

For treating intestinal worms, Noga recommends dosing fenbendazole into a QT at 2 mg/L (7.6 mg/gallon) once/week for 3 weeks. However, you can also try lacing fish food with fenbendazole. (DO NOT dose or food soak fenbendazole in a reef environment.) You’ll need to use a binder (Seachem Focus, unflavored gelatin, Agar) to reduce the loss of medication to the water through diffusion. My simple recipe* for food soaking dewormers can be found below:

• 1 tbsp food (preferably frozen food or pellets)
• 1 scoop (~ 1/8 teaspoon) of medication
• 1-2 scoops of Seachem Focus (binder) OR unflavored gelatin or agar are other binding agents that can be used.
• A pinch of Epsom salt to help expel dead worms/parasites
• Several drops of saltwater or fish vitamins to wet everything down
• Stir until a medicated food slurry has been achieved
• Feed after soaking for 30 mins
• Refrigerate or freeze any leftovers for future use
• Feed daily for 2-3 weeks

* The “proper way” to food soak medications is by weight. (Some sources say to weigh the food, some even say to weigh the fish. ) I find this to be impractical for the casual hobbyist, and that my “shotgun approach” to food soaking dewormers generally works. 

Pros: Highly effective dewormer

Cons/Side Effects:

• Some fish will act “off” for 24-48 hours after treating with Fenbendazole and may not eat.

• However, @Dierks reports that wrasses are the only fish he has ever lost after treating with Fenbendazole.

• Fenbendazole is NOT reef safe. Not even when soaked in food.

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