Dr. Fish
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Navarre, FL
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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 TreatFenbendazole 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 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)
  • 1 tbsp food (preferably pellets or frozen food)
  • 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.

ProsEffective dewormer that is relatively gentle on most fish.

Cons/Side EffectsFenbendazole is NOT reef safe. DO NOT food soak or dose it into a reef environment. At the time of this writing, fenbendazole is a relatively new medication, so all the downsides/side effects are probably not yet fully understood.
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Also note that if you are going to do the bath and dissolve the Fenbendazole be careful with Wrasses. If you do have a fish that does not take the Fenbendazole bath well I personally have had success with the Methylene Blue Bath to detox them.


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We used it at first with focus in the DT and believe that it eventually killed a leather a month after first using even though there was only a few feedings, then moved fishies to a QT. We have a mushroom that totally consumed food with bendazole and prazi pro after it landed on it and it’s still alive. We also believe that a small acan colony looked like it was dying but appears to be bouncing back 3mth later.

I think our fish would still have intestinal issues if it wasn’t for this because GC didn’t appear to be working. In the qt tank we did not use focus and fish seemed more eager to eat food and didn’t want anything to do with focus.
Never dosed tank only mixed with food.


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I have used fenbendazole in a reef. It will kill all octocorals. Does a great job on getting rid of pesky Xenia, GSP and other octocorals. It also kills bristle worms, some snails (I don't remember which ones live and which ones die so remove all) and it also kills hydroids. The tank had SPS and LPS and the dreaded pest Xenia. It has no effect on the SPS and LPS. If the tank is mature expect to see a massive die off of bristle worms and be ready to vacuum them out daily if not more.

I used powerhead to blast the rock and sand to get the fenbendazole into the water stream so that it could be removed with micro filtration and carbon. I kept the snails out for 2 months and reintroduced with no issue.

If you like your octocorals, do not use. If you want to rid you tank of these, it will work as stated. It takes several days to see the effect, but be ready for ammonia and nitrite spikes.