Public aquarium accidentally kills 100 reef fish with botched fluke treatment

by | Oct 3, 2014 | Advanced Aquarist | 0 comments

What is Dylox and praziquantel?

Dylox is the organophosphate, trichlorfon.  Trichlorfon is one of the active ingredients in many over-the-counter fluke/worm medications.  While it has been shown effective against monogenetic trematodes (flukes), some monogeneans have developed resistance to this organophosphate.  Dylox is also a  neurotoxin that can cause serious harm to animals including humans.  Fish exposed to high levels of Dylox (or old batches of Dylox) can exhibit nervous system and respiratory problems leading to death.  Even low dosages of Dylox can visibly stress fish.  For these reasons, it has fallen out of favor as a first-option fluke treatment.

On the other hand, praziquantel is extremely gentle on fish and rarely causes significant distress or death.  Overdosing with praziquantel almost requires willful intent because very large dosages are required to harm fish.  It’s also proven to be a highly effective anti-trematode treatment for both freshwater and saltwater fish, although BioPark Aquarium staff reported praziquantel failed to cure their fish of flukes.

Learning from Tragedy

This unfortunate event reminds aquarists that when improperly administered, the cure can sometimes be worse than the disease.

  • Research your medication.  Know what medication is useful for what ailment, and just as importantly, which medication is best suited for the species of fish you intend to treat.  Some medications do not work well with specific fish (e.g. praziquantel and some loaches, copper and some angelfish).
  • Carefully calculate dosage.  Double and triple check your decimal points and conversions!
  • Have emergency water-change water on hand.
  • Do not mix medications unless you are absolutely certain there are no contraindication.
  • Never use old or expired medication. For example, old Dylox can be particular harmful to fish.
  • It is advisable to start treatment with the most proven, gentle medication first before advancing to more aggressive treatments as necessary.
  • Weigh the risks versus reward when using any harsh chemical treatments.


  • Leonard Ho

    I'm a passionate aquarist of over 30 years, a coral reef lover, and the blog editor for Advanced Aquarist. While aquarium gadgets interest me, it's really livestock (especially fish), artistry of aquariums, and "method behind the madness" processes that captivate my attention.


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