This is tapeworm, commonly known as noodle worm. Sick fish are generally thin, with swollen abdomen, severe atrophy of internal organs and damage to liver and kidney. Scattered in the worm, the intestine is as thin as a line. Treatment: oral administration, fenbendazole and VC stress spirit are added to the feed, and antibacterial drugs such as carbendazim and Kejunling should be added for 3-4 days after intestinal mucosa damage; Spraying trichlorfon on the water body and using 200-330g crystal trichlorfon at the water depth of per mu 1m can kill Daphnia in water and cut off the life cycle of this insect.