Lice in Freshwater Fish

Fish lice Argulus genus on perch

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Freshwater fish lice are members of the Argulus genus. These macroscopic parasites are visible to the naked eye and look like grey, brown, or green spots on your fish's skin. Once diagnosed by your aquatic veterinarian, treatment needs careful consideration to make sure the lice are gone for good.

What Are Lice in Freshwater Fish?

Lice cause skin irritation and can make your fish very itchy. They can spread very quickly in warm water and overwhelm your fish.

Symptoms of Lice in Freshwater Fish

  • Pin hole to pencil eraser-sized grey or green dots on your fish's skin
  • Dots all over fish or condensed at fin attachments
  • Flashing or fish itching themselves on substrate and décor items
  • Bruising or missing scales secondary to flashing
  • Increased lethargy
  • Decreased appetite

Causes of Lice

There are many species of Argulus lice, depending on your location, species and water temperature. It only takes one male and one female louse to quickly start replicating.

Lice will enter your system undetected on an infested fish. If not properly quarantined, your fish and its low lice load will quickly spread throughout your system. Juvenile lice are microscopic and may not be noticed until they grow into adults. By holding new fish in quarantine, you will catch the adults before they spread and treatment will be significantly simpler.

Diagnostic Process

Although lice are macroscopic, you will need a qualified aquatic veterinarian to take samples to make sure no additional parasites or disease processes are present. Lice will take advantage of weak fish and can replicate very quickly, particularly in warmer water systems. Lice irritate your fish's skin and can provide an entry point to other parasites and bacteria.

In working up a potential lice case, your veterinarian should address your system's water chemistry and your fish's diets. The worst fish stressor, poor water quality, can weaken your fish's immune function and allow lice outbreaks to happen more rapidly. Feeding a poor diet can also weaken your fish's defenses. In treating any disease outbreak, proper care and support through water quality and diet will greatly impact your fish's chance at a full recovery.


Depending on your state's regulations, various freshwater lice treatments will be available. If your fish is severely covered in lice, manual removal under sedation by a veterinarian will be the first step. Water-based treatments may be prescribed from that point in order to treat the entire system. Treatment duration will vary based on the temperature of your water. Complete diagnostics for any additional disease processes are critical in order to correctly time medication applications.

Some freshwater fish lice treatment may also kill off other invertebrates and bugs in your tank or pond. You may see a slight ammonia spike from this die off process after adding medication.

How to Prevent Lice

The simplest way to treat lice is to keep them out of your system. You should quarantine all new fish and invertebrates for 4-6 weeks before adding them to a pre-existing system. This is the only way to guarantee that no harmful diseases or parasites will harm your current tank or pond occupants. Proper quarantine setups will require a tank or pond out of splash range of your current system and all separate equipment. Closely monitor your new additions, and if you notice something amiss, begin treatment or contact your aquatic veterinarian for correct diagnosis and effective treatment protocols. Your patience will be rewarded and your fish will thank you.

Live freshwater plants should be quarantined for 2 weeks to break any parasite life cycles if they are carrying lice. Unless you can 100% verify that your plants come from sources with no fish, use the quarantine period to enrich your new plants with fertilizer and prepare them for going into a new system after the stress of transport.