Ichthyophthirius multifiliis

Ichthyophthirius multifiliis, commonly known as "ich," is a parasitic protozoan that infects freshwater fish, particularly in tropical aquariums. It causes small white spots on the fish's body and gills, leading to loss of appetite, abnormal behavior, and potentially death due to tissue damage and secondary infections.1

The parasite matures within the fish's skin, making it resistant to water treatments. It then leaves the host to form a protective cyst, releasing immature forms called tomites that must quickly find a new host to survive. Copper sulfate can be an effective treatment at specific stages of the parasite's life cycle.1

Hymenostomes are a group of protozoa within the order Hymenostomatida, characterized by a ventral mouth cavity with fused cilia. This order includes well-known genera like Paramecium and Tetrahymena, often used in research, as well as parasites like Ichthyophthirius.2