Warning: DO NOT use with wrasses!

Fenbendazole (dewormer)

What It Treats – Flukes (Monogeneans), and gastrointestinal parasites (worms). Particularly useful to treat strains of flukes & intestinal worms which are resistant to praziquantel.

How To Treat – Fenbendazole is a drug often prescribed by veterinarians to treat intestinal parasites in dogs & cats (sometimes sold as Panacur). It is also effective to use on fish, and can be purchased online in bulk (click here) or is contained in the aquarium medication Thomas Labs Fish Bendazole.

For treating intestinal worms (white stringy feces), you can dose fenbendazole into a QT at 2 mg/L (7.6 mg/gallon) once/week for 3 weeks. However, it is probably best to food soak the medication so it passes through the gastrointestinal tract where the worms live. You’ll need to use a binder (e.g. Seachem Focus, unflavored gelatin, Agar) to reduce the loss of medication to the water through diffusion. I recommend feeding fenbendazole for 2-3 weeks, or until symptoms (white stringy poo) are no longer present. My recipe for food soaking medications can be found below:

Using a shot glass:
  • 1 tbsp food (preferably pellets or frozen food)
  • 1 scoop (~ 1/8 teaspoon) of medication (Fenbendazole)
  • 1 scoop Seachem Focus (this helps to bind the medication to food – but Fenbendazole is NOT reef safe)
  • A pinch of Epsom salt to help expel dead worms/parasites
  • A few drops of saltwater or fish vitamins
  • Stir until a medicated food slurry has been achieved
  • Feed after soaking for 30 mins
  • Refrigerate or freeze any leftovers for future use

Noga also 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). It would be advisable to transfer the fish into a sterile QT following the bath treatment (to prevent reinfection), and to perform a second bath + transfer 1 week later to eliminate any hatchlings. (Same as using praziquantel.)

At the time of this writing, it is still unclear just how “safe” prolonged immersion of fenbendazole is at 95 mg/gallon. Meaning, can it be used the same as praziquantel in a QT: Dosed once, and in 5-7 days perform a 25% water change and dose again (to eliminate eggs/hatchlings). The lower 7.6 mg/gallon dosage appears safe for in-tank use, but is that concentration strong enough to kill skin & gill flukes? One issue with fenbendazole is that it’s not easily water soluble, and turns the water cloudy/milky for at least 12 hours (see below). Using a drop or two of Dimethyl sulfoxide (DMSO) will help to fully dissolve Fenbendazole in a cup before dosing.

Pros – Effective dewormer that is relatively gentle on most fish.

Cons/Side Effects – Fenbendazole is NOT reef safe. DO NOT use with wrasses! At the time of this writing, fenbendazole is a relatively new medication, so all the downsides/side effects are probably not yet fully understood.

Edit: Here’s some really good info on exactly what fenbendazole does and doesn’t affect in a tank: https://seahorse.com/faqs/fenbendazole-panacur/