Lice Treatment for Freshwater Fish

Fish lice Argulus genus on perch

The Argulus genus includes freshwater fish lice. These macroscopic parasites are visible to the naked eye and appear on your fish's skin as grey, brown, or green patches. Once your aquatic veterinarian has detected lice, treatment must be carefully considered to ensure that 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 d├ęcor 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.

On a contaminated fish, lice will enter your system unnoticed. Your fish and its modest lice burden may soon spread throughout your system if not properly quarantined. Because juvenile lice are so little, they may not be recognized until they mature into adults. Holding young fish in quarantine allows you to catch adults before they spread, making treatment much easier.

Diagnostic Process

Although lice are microscopic, you will need to have samples taken by a skilled aquatic veterinarian to ensure there are no other parasites or disease processes present. Lice will prey on weak fish and may reproduce fast, especially in warmer water environments. Lice irritate your fish's skin and can function as a gateway for parasites and pathogens.

When investigating a possible lice infestation, your veterinarian should look at the water chemistry of your system as well as the diets of your fish. Poor water quality, the most serious fish stressor, can damage your fish's immune system, making lice outbreaks more likely. A bad diet might also compromise your fish's defenses. In the event of a disease outbreak, effective care and support via water quality and nutrition will have a significant impact on your fish's ability to recover completely.


Various freshwater lice treatments will be offered according on your state's restrictions. Manual removal under anesthesia by a veterinarian will be the initial step if your fish is seriously infested with lice. From there, water-based therapies can be recommended to treat the entire system. The length of your treatment will be determined by the temperature of your water. In order to time medicine administrations accurately, complete diagnoses for any additional disease processes are required.

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

Keeping lice out of your system is the easiest method to cure them. All new fish and invertebrates should be quarantined for 4-6 weeks before being added to an established system. This is the only method to ensure that your present tank or pond inhabitants are not infected with hazardous illnesses or parasites. A tank or pond out of splash range of your present system, as well as any separate equipment, are required for proper quarantine setups. Keep a close eye on your new recruits, and if you detect anything unusual, start treating right away or call your aquatic veterinarian for a proper diagnosis and treatment plan. Your patience will pay off, and your fish will appreciate it.

If live freshwater plants possess lice, they should be isolated for two weeks to disrupt any parasite life cycles. Unless you can 100% guarantee that your plants came from fish-free sources, utilize the quarantine time to nourish your new plants and prepare them for a new system after the stress of transportation.