Ichthyophthirius multifiliis (commonly known as freshwater white spot disease, freshwater ich, or freshwater ick) is a common disease of freshwater fish.It is caused by the protozoan Ichthyopthirius. Ich is one of the most common and persistent diseases in fish.The protozoan is an ectoparasite.White nodules that look like white grains of salt or sugar of up to 1 mm appear on the body, fins and gills. Each white spot is an encysted parasite.t is caused by the protozoan Ichthyopthirius. Ich is one of the most common and persistent diseases in fish. The protozoan is an ectoparasite. White nodules that look like white grains of salt or sugar of up to 1 mm appear on the body, fins and gills. Each white spot is an encysted parasite.