Warning: DO NOT use with wrasses!
Fenbendazole (dewormer)

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:
- 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.)

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/