Lice in Freshwater Fish

Causes, Treatment, and Prevention

Fish lice Argulus genus on perch

Henrik_L / Getty Images

Freshwater fish lice are macroscopic parasites that present as small spots on your fish's skin. Fish lice are highly infectious and can be very dangerous to freshwater fish because of the parasites they carry. Symptoms of fish lice include lethargyabnormal swimming patterns, and itchy skin. To diagnose a fish with lice, look carefully at its skin for spots visible to the naked eye. Immediately visit your vet for treatment options before the infestation worsens. The prognosis is good for infestations treated early, but anti-louse medications aren't always readily available. 

What Are Lice?

Fish lice (Argulus) are a species of parasitic crustaceans that can affect freshwater fish. The louse is flat, oval, with a thin, needle-like sucker used to pierce the fish to which it is attached. Fish with fish lice have a high mortality rate, mainly because the lice can carry other parasites that lead to deadly secondary infection. In addition to viruses passed through the louse, bacterial and fungal infections can occur at the puncture site. Fish lice outbreaks pose problems to freshwater fisheries, sometimes necessitating season-long shutdowns while the lice are eradicated.

Symptoms of Lice in Freshwater Fish

Lice can cause uncomfortable symptoms in your fish, but sometimes, there are no symptoms except the presence of the parasite.


  • Dots on fish
  • Scratching
  • Lethargy
  • Abnormal swimming

Dots on Fish

Typically, small, red dots will appear at the site of the louse infection. The red dots result from tiny hemorrhages, inflammation, or irritation from digestive enzymes caused by the parasite puncturing the fish's flesh.


Lice can be very itchy. If you notice your fish scratching against hard surfaces in its tank to relieve the itch, losing fins or scales in the process, your fish may have lice.


As a result of a lice infestation, your fish may act lethargic and swim less than usual. This is due to anemia brought on by the lice. The loss of blood caused by the parasite can lead to lethargy.

Abnormal Swimming

Fish with lice may swim abnormally due to discomfort, or in an attempt to shake the lice off of its body.

Causes of Lice

Fish lice infestations happen easily and are affected by several factors.

  • Environment: A fish's environment affects its likelihood of contracting lice. Most commonly, fish lice inhabit freshwater, but they also live in marine environments.
  • Time of day: Fish lice are more active in darkness and use their sense of smell to detect fish.
  • Hosts: A variety of hosts are responsible for transmitting fish lice. Mainly, fish lice have been inadvertently spread on koi fish and goldfish throughout the United States. Frogs and toads can also transmit fish lice.

Diagnosing Lice in Freshwater Fish

Fish lice are macroscopic, meaning they can be identified with the naked eye. Once you notice the lice, bring the fish to a vet for a closer examination and identify any other possible parasites at play. Your vet may take impression smears from skin scrapes, biopsy the fish, or perform a nasal flush.


There are many options for the treatment of fish lice. In some cases, a vet can manually remove the lice from the fish, but this can be risky to the fish and result in an incomplete removal. Only the adult parasites are removable, so lice in other life stages will likely remain on the fish's skin and in the tank. You should eliminate any eggs remaining in the tank with complete disinfection. Medications are available to treat fish lice, but they are limited and not approved by the FDA. The best way to treat fish lice is through low-dose insecticides. Do not begin any treatment without consulting your veterinarian. 

Prognosis for Freshwater Fish With Lice

Your fish's recovery from fish lice will depend on the severity of the infestation at the point of treatment. Fish that haven't suffered from secondary infections should be able to recover fully with appropriate treatment.

How to Prevent Lice

Even with safety measures, fish lice can be difficult to prevent. It is essential to screen and quarantine fish for four to six weeks before adding them to an existing system. Additionally, you should quarantine live freshwater plants for two weeks, as they may also carry lice. This is the only way to guarantee that no harmful diseases or parasites will enter your current tank or pond.

  • What do fish lice look like?

    Fish lice look like little red dots on your fish's skin.

  • Are fish lice treatable?

    Fish lice are treatable, but the success of the treatment depends on the severity of the infestation and the drugs available to you. There is no FDA-approved medicine, so treatment can sometimes be complicated.

  • Are fish lice dangerous?

    Fish lice are dangerous, mainly because they often carry parasites of their own, leading to secondary infection in fish. 

Article Sources
The Spruce Pets uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. ARGULUS (FISH LOUSE) INFECTIONS IN FISH. University of Florida.
  2. Fish Lice (Argulus japonicus) in Goldfish (Carassius auratus). National Library of Medicine.
  3. Argulus. Science Direct.