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About Costiasis

Costiasis or Costia is a freshwater ciliated protozoa parasite species called Ichthyobodo necatrix or Ichtyobodo pyriformis. Formally known as species Costia necatrix.

These are stalked or Flagella (hairs) protozoa which attach to the skin or gills of fish to feed and can swim and attach themselves to other fish and then reproduces to infect other fish.

Quite a common parasite in Koi or outdoor Goldfish.


Signs of infection include excess mucous production, respiratory distress, lethargy, clamped fins, general debilitation and resting on the substrate.

The parasites feed on the gill and skin cells and can quickly kill fish by depriving them of oxygen and allowing secondary infection of fungus and bacteria in to the wounds.

  • This parasite can multiply quickly so treatment needs to be rapid.


  • The Costiasis parasite may be becoming salt resistant and is not killed by drying out equipment. So use a decent disinfectant on nets, hands, etc. to prevent parasite transmission between tanks.[1]
  • You will need 200X minimum magnification to spot Costia. Preferably 400X if infection is low.


  • Due to its inability to live in water above 28, treat as if it was Ich by using a commercial Ich treatment or technique.

Commercial treatments

King British



Pets at Home

  • None

API (Aquarium Pharmaceuticals)

Tetra UK

Tetra USA

Jungle Labs (USA)


  • Proform C - Malachite Formalin mix for parasite control in ornamental systems and ponds (contains Malachite green and formalin)


  1. Koivet on the Costiasis parasite

External links