Used normally, with Malachite green, and this aids the eradication of the above mentioned parasites it strips off some to the protective mucus layer on the fish thus allowing the Malachite Green to work ...
Malachite green and Formaldehyde are the best treatment for common external parasitic infections.
Never add Formalin to a pond which has a salt content.
Formalin is a very active chemical and is best used with Malachite for the effective treatment of most single cell parasites including white spot, costia, and some fluke
Infestations. We do not recommend the use of Formalin with any salt in solution. For use above 10c (50f) dosage 75mls per 1000 gallons.