Costia Necatrix is one of the most serious parasites in goldfish. If not treated immediately, Costia can be a fast killer as the parasite attacks the fishes gills. Costia can live in cooler waters, which can pose a large threat in the spring time when the fishes immune system is weakened, while the parasite is at full strength.


Symptoms include; 

  • Gasping at the surface of the water
  • Slimy patches around the head and gills
  • Sudden death (especially in spring)
  • Small hemorrhages under chin
  • Spider web kind of lesions


To treat Costia, the bes, and most gentle solution is... salt. A 0.3% salt solution is recommended. Typically a 0.1% solution is added to the tank every 12 hours for 36 hours (or 3 times). A 0.1% solution is made up of 1tbsp salt to 5gl of water.

If the salt solution does not do the trick other medications are necessary. The best medication is QuickCure, which contains Formalin. Potassium permanganate has also been found effective.