Gill or Skin flukes

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These are common worm like parasites (or ectoparasite) about 1-2mm in length which live in and reproduce in the gills of cold water or tropical fish (occasionally on to the surrounding skin). Due to their small size they are hardly visible.

Indeed it is sometimes said that "Flukes are to fish as fleas are to dogs". Many aquarists treat for flukes on getting any new fish as a matter of routine.

They are called monogenean trematodes, which describes their biological classification (trematodes) and the fact they only need one host to complete their life cycle (monogenean).

A gill fluke has a series of hooks on its body and uses them to attach and drill into the flesh of the fish via the gills. This leaves holes in the flesh which can be lead to secondary infection by bacteria. But the flukes also reduce the ability of the fish to take in oxygen and if the flukes multiply then the fish becomes listless and dies of either a bacteria infection or oxygen starvation. Usually very quickly indeed.

Life cycle[edit]

The worm lives for about a week and drops eggs which hatch out with 2 to 4 days. If the larvae doesn't find a host it dies.

The two main species are:


This species are the parasite commonly called gill flukes.


Whilst Gyrodactylus are often said to be skin flukes either species will attack the gills.


Yes. Very. Treat the entire tank or remove all fish and treat them in a quarantine tank. In that case leave the main tank empty of fish for at least 7 days.

External symptoms[edit]

Initially you'll see rubbing and flashing of the fish. Heavy infestations are life threating as bacteria and fungus infections often occur. As the parasite reaches a more advanced stage the fish will become lethargic. As the parasite multiplies the fish will isolate itself and spend long periods laying on the bottom with its fins clamped to its body or simply keeping to a corner of the tank.

Often you'll see rapid breathing, loss of appetite, discoloured gill filaments, and swollen gills. Infected fish may also gasp at the water surface where the water has a higher oxygen concentration or lay at the bottom due to the gill damage.

If you can observe both sides of the fish, you may see that a gill cover is clapped shut or permanently open. Other signs may be skin cloudiness resulting from excess mucus production.

A definite diagnosis can only be made via a skin scrape or gill biopsy. In large numbers, flukes will kill fish either directly, or indirectly through secondary infections. So you should treat afterwards with a general bacteria treatment.


There are numerous gill fluke treatments available that are added to the water. However their eggs are hardy and most treatments only attack the young newly hatched flukes so several treatments are often needed over several weeks.

  • Praziquantel (also called Droncit (USA) or Fluke-Solve (UK/EU), Drontal (UK), PraziPro (Hikari)) - Preferred method for all fish whether salt or freshwater due to effectivity and safety.
  • Fluke tabs - This is very strong and may harm or stress your fish. Read the instructions first. Not recommended these days.
  • Paragon - by Waterlife UK (Mild infections) (safe with shrimps and snails).
  • Sterazin - by Waterlife UK (Serious infections) (not safe with shrimps and snails).

Note: If there is a large amount of dissolved organic material in the aquarium then the treatment may need to be repeated after 7 days as it will attempt to breakdown this material as well as the parasites.