Ichthyophthirius multifiliis is a single-celled parasite commonly known as "ich." It primarily affects freshwater fish, causing a disease called "ichthyophthiriasis." The parasite attaches to the fish's skin, gills, and fins, leading to symptoms like white cysts, irritation, and difficulty breathing. This organism thrives in warm water and can spread rapidly in crowded or poorly maintained aquariums. Treatment often involves raising the water temperature and using medications specifically designed to eliminate the parasite. Proper tank maintenance and water quality are essential to prevent outbreaks of ich.