Leech on the back of a fish

Leeches are not a very common parasite in most captive-bred, freshwater aquarium fish. If you are bringing in a wild species, however, you will need to keep an eye out for leeches, in addition to other wild parasites. Correct identification by a veterinarian is required for effective treatment.

What are leeches?

Leeches are typically a type of worm that attaches to the skin of an animal and feeds on blood of their host. Members of the subclass Hirudinea within the phylum Annelida, 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

  • Leeches hanging off fish's body, in oral cavity or gill cavity
  • Pale gills
  • Lethargy
  • Increased respiration
  • Negative buoyancy

Small leeches may not be visualized until they grow in size. Any raised, black lines on your fish may be a normal color, traumatic wounds, or a tiny leech. A hands-on physical exam from your veterinarian will determine if the color defect is normal or a dangerous pathogen.

Loss of blood can be indicated by many clinical signs including pale gills, lethargy, increased respiration, and hanging around oxygen sources, such as filter outflows or air stones. Anemia in fish can be caused by many different causes, the most common being diet deficiencies.

Buoyancy disorders are very common for many causes of stress. Negative buoyancy disorders are common in low energy fish, when they cannot take in enough nutrients or oxygen. With leech infestations, blood loss is the main contributor to lethargic, negatively buoyant fish, simply due to the fact that they lack the energy to swim.

Causes of Leeches

Leeches will enter the systems of an infested individual who is not properly quarantined. They may hide within the opercular or oral cavity of a fish, making them hard to note at first. In order to reproduce, there has to be two leeches present. Although they are hermaphroditic, they start their lives as males, with ovaries growing later in life. Unlike some other types of worms, leeches can only reproduce sexually and cannot regenerate pieces of their body.


If you note a leech on your fish, physical removal as soon as possible is the best treatment. If you are comfortable handling your fish, proceed slowly and use two nets to quickly and efficiently capture your fish. Keep them in the net and use a pair of tweezers to remove the leech. Once the leech is removed, it is critical that it is removed from the system or it will attach to another fish. Be forewarned that if you squeeze the leech, a large amount of blood may gush out of it!

If you are not comfortable or unsure about handling your fish, call your local aquatic veterinarian. They will safely capture and sedate your fish for easy leech removal. During a sedated physical exam, your veterinarian will be able to examine all orifices of your fish to make sure there are no additional leeches on your fish, specifically in the oral cavity or gills.

How to Prevent Leeches

Leeches on fish are very rare for all captive bred fish. Wild-caught fish are the most likely to be carrying leeches. Try to prevent taking any fish from the wild in order to protect wild stocks. If you catch fish from your local stream or lake, please replace them in their homes, and do not bring them home with you.

If you must 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 very close eye on your new additions and if anything starts to seem amiss, a swift diagnosis and effective treatment are the best bet for your fishes’ survival. It is highly recommended to have the contact information of your local aquatic veterinarian handy just in case you have questions or need help quickly.

Are Leeches Dangerous to Humans or other Pets?

Leeches are not very discriminative parasites and can easily attach to humans or other pets. Once you remove a leech from your fish, crush the head with the tweezers you used for removal. Keep in mind that there may be a gush of blood when you do this. Fish blood is not harmful to humans or other pets, but be sure to wash contact sites well with soap and water. If you do not kill the leech, there is a chance it may attach itself to you, another unsuspecting human, or one of your other pets.

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