How to Treat Fish With Internal Parasites

Updated November 21, 2016

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.

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.

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

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.

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

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.


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.

Things You'll Need

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