Ichthyophthirius multifiliis

Ichthyophthirius multifiliis, commonly called "Ich" or "fish lice," is a parasitic protozoan that infects freshwater fish. It causes white spots or lesions on the fish’s skin, fins, and gills, leading to irritation, breathing problems, and sometimes death if untreated. The parasite has a life cycle involving a free-swimming stage that seeks out fish to infect, where it burrows into the skin. It is highly contagious but treatable with specific medications and good aquarium management, including maintaining clean water and quarantining new fish to prevent outbreaks.