Leeches in Freshwater Fish

Causes, Treatment, and Prevention

Leech on the back of a fish

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Leeches are not a very common parasite in most captive-bred pet fish. But, if you introduce wild fish to your tank, then leeches may tag along. These worm-like organisms attach to fish bodies, particularly the mouth and gill areas, and feed on blood. This causes paleness and lethargy in their hosts. Correct identification by a veterinarian is required for the eradication of leeches.

What Are Leeches?

Leeches are parasitic worms that attach to the skin of host animals and feed on their blood. They are close relatives of the earthworm. Most leeches live in freshwater, but there are a few species that can be found in marine systems.

Symptoms of Leeches in Freshwater Fish

Small leeches may not be noticed until they grow in size, but even small leeches in large numbers can affect fish by draining their blood. Symptoms of these parasites include:


  • Leeches present on one or more fish
  • Pale gills
  • Lethargy
  • Increased respiration
  • Negative buoyancy

Loss of blood can be indicated by pale gills, lethargy, increased respiration, and the urge to hang around oxygen sources, such as filter outflows or air stones. Anemia in fish can have many different causes, the most common being diet deficiencies, but leeches may also be the culprit.

Negative buoyancy disorders are common in low-energy fish, especially when they cannot take in enough nutrients or oxygen. With leech infestations, blood loss causes fish to be lethargic and negatively buoyant because they lack the energy to swim.

Causes of Leeches

Leeches are introduced to aquariums via one or more infected fish that have not been properly quarantined.

  • Young leeches may hide within the opercular or oral cavity of a fish, making them hard to see.

To reproduce, there must be two leeches present. Unlike some other types of worms, leeches can only reproduce sexually and cannot regenerate pieces of their body. Unfortunately, multiple leeches will usually be present on a single fish, making reproduction likely.

Diagnosing Leeches in Freshwater Fish

If you are unsure about the identity of the parasites on your fish, consult a veterinarian who is familiar with freshwater aquarium fish and will be able to recognize leeches on sight.


If you see a leech on your fish, prompt removal is the best treatment. If you are comfortable handling your fish, capture and hold the fish in a net while you use a pair of tweezers to remove the leech. Once removed, dispose of the leech outside the aquarium so that it does not attach to another fish. Be forewarned that if you squeeze the leech, a large amount of blood may gush out of it. Fish blood is not harmful to humans or other pets, but be sure to wash contact sites well with soap and water.

If you are not comfortable or unsure about handling your fish, call your local aquatic veterinarian.

Prognosis for Freshwater Fish with Leeches

Once leeches are eradicated from your aquarium, your fish should fully regain their health.

How to Prevent Leeches

Leeches are very rare in captive-bred fish. Wild-caught fish are the most carriers of leeches. If you purchase wild-caught fish, quarantine all new individuals for 4-6 weeks in a separate, isolated system. Use separate equipment for all systems, and do not allow water from one system to splash into another.

Keep a close eye on your new fish and treat problems that arise before introducing them int your main aquarium.