30-05-2022, 18:26
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Flukes (Trematoda)

Gill flukes (Dactylogyrus) and skin flukes (Gyrodactylus) are common parasitic flatworms that affect fish, frogs and turtles. Despite their name, both trematodes can be found on the skin and gills of cold-water and tropical fish, where in small numbers they do not harm healthy fish. In large numbers, the sosalcids (trematodes) are debilitating and often cause secondary bacterial infections.

There are more than 10,000 species of trematodes worldwide. They range in size from 0.5 to 10 cm in length and infect domestic animals, humans, shellfish, and almost all tropical fish.  The flukes can attach externally, internally (in the gut), and semi-externally in the gills and mouth of fish.

The gill flukes (Dactylogyrus) uses hooks on its body to attach to the flesh of fish gills. As it reproduces, the fish's ability to absorb oxygen decreases, the fish become sluggish and die from oxygen deprivation or from bacterial infection.

Gill flukes (Dactylogyrus) live for about a week and lay eggs that hatch after 2-4 days. 


Gill and skin suckers are small, flat, white worm-like parasites that are difficult to detect without the aid of a microscope.

Symptoms include the following:

Gill fluke (Dactylogyrus):

  • Loss of appetite
  • Fish becoming lethargic
  • Irritated, discolored, injured gills
  • Heavy breathing at the water surface


Skin sucker (Gyrodactylus):

  • Fish rub on different surfaces
  • Tiny red spots on the body
  • The fins clamped to the body
  • Cloudy coloration due to a large amount of mucus

In the later stages of Gyrodactylus infestation, the fish become sluggish and spend a long time at the bottom of the tank with their fins pressed against their body.

Dermal sucker (Gyrodactylus)

In the late stage of Dactylogyrus infestation, fish often have swollen, discolored gills and lose their appetite. Gill lids may be compressed or remain wide open, and the fish's skin may be cloudy due to excessive mucus production.

Severely infested fish usually grab air with their mouths at the surface, where the water has a higher concentration of oxygen, or lie on the bottom, suffering severe gill damage.

Severe infestations kill the fish directly or through secondary infections.


Sopales (Trematoda) are caused by ingestion of infected fish or plants carrying the parasite's eggs or adult parasite into the aquarium.


Of the many treatments available for both livebearing and spawning fish, those containing praziquantel are the most effective.

Most louse treatments are directed at the juvenile and adult stages, which is satisfactory for Gyrodactylus, but gill louse (Dactylogyrus) and other egg-laying trematodes require several treatments over several weeks to kill the hardy egg offspring. There is no fish-friendly treatment that will kill the eggs of the caterpillars, therefore repeated treatments are necessary to break the life cycle completely.

If there is a large amount of dissolved organic matter in the aquarium water, repeated treatments are definitely required after 7 days.

Most tropical fish aquarists use praziquantel as a safe and effective treatment for cutaneous trematodes (Gyrodactylus sp.), gill trematodes (Dactylogyrus sp.) and tapeworms in koi, goldfish and tropical fish. Although praziquantel is mainly used to treat parasitic infections in humans and animals, it is also used in aquariums.

Praziquantel is generally safe for fish, aquatic plants, and does not affect biological filtration in the aquarium.

Praziquantel is a white powder that is difficult to dissolve in water. 

Aquarists suggest an easy way to dissolve Praziquantel using a nylon stocking. Simply place the desired dose in the stocking, tie it up, immerse it in the aquarium, and gently roll it between your fingers until it dissolves. The dissolved Praziquantel slowly comes out of the stocking as a white cloud.


Poor water quality and high temperatures accelerate the reproductive cycle of the salsapods (trematodes), and since the life cycle of monogenic salsapods does not require an intermediate host, they can remain in a closed aquarium environment almost indefinitely.

The best prevention of trematode infestation is to quarantine all fish and plants before putting them in the aquarium. Since the shovelnose worms are microscopic and difficult to detect visually, all new plants and aquarium inhabitants must be closely monitored.

Maintaining proper water quality and controlling waste levels in the system is important to reduce existing bivalve populations and ensure a healthy immune system for the fish.

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