How to Treat Fish With Internal Parasites

Written by sarah goodwin-nguyen
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You or your veterinarian may suspect internal parasites in your fish if you see signs of listlessness, gaunt appearance, a bloated abdomen, or white worms in the fish's anus. Common endoparasites (parasites that live in the tissues, blood or organs) include digeneans, or flukes; threadworms and roundworms (nematodes); and tapeworms, or cestodes. Praziquantel is the drug of choice for treating internal parasites in fish. Praziquantel powder is effective for both marine and freshwater species.

Skill level:

Things you need

  • Quarantine tank
  • Praziquantel powder
  • Vodka (ethyl alcohol)

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  1. 1

    Set up a quarantine tank for medicating the infected fish. This can be a 10-gallon to 20-gallon tank with a hang-on-back or canister filter, a hiding spot or two and an aerator. Use water of the same temperature and pH as the aquarium from which the fish came to prevent shock and stress.

  2. 2

    Place infected fish in the quarantine tank. Turn off your filtration, but leave an aerator running.

  3. 3

    Dissolve praziquantel powder in vodka (ethyl alcohol), following your veterinarian's prescribed dosage or the manufacturer's directions for the formulation you have purchased. Add the prazi-vodka solution to your quarantine tank.

  4. 4

    Turn on your filtration after three to six hours. Water changes are not necessary with praziquantel unless your fish is showing signs of stress.

  5. 5

    Repeat the treatment based on the life cycle of the suspected parasite to ensure that any newly-hatched parasites are destroyed. A course of three treatments over 10 to 14 days is usually effective.

Tips and warnings

  • According to National Fish Pharmaceuticals, to become water-soluble praziquantel must be combined with alcohol.
  • You can use praziquantel to treat an infected freshwater tank without harming plants.
  • Praziquantel is also available as an oral (feed) or injectable medication.
  • Praziquantel is not recommended for use on coral or other invertebrates.

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