Ichthyophthirius multifiliis, also known as "ich" or "white spot disease," is the best known of the parasites affecting freshwater tropical fish. Ich can infect most species of freshwater fish and multiplies rapidly, causing devastating and rapid death within an aquarium. Ich feeds on the skin and gills of fish, providing sites for infection by other organisms, such as bacteria and fungus. In addition, the resulting "holes" in the outer layers of the fish make it difficult for the fish to maintain the proper concentration of salts in its body.
For more information, please read Ich-White Spot Disease.