Ichthyophthirius multifiliis
The disease starts when the parasite digs into the skin of the fish. This causes swelling and white spots to form. After a few days, the parasite turns into a trophozoite, a jelly-like substance. This falls off the fish and lives on the bottom of the tank. Inside the jelly are hundreds of baby parasites called tomites. These eventually can swim by themselves and go looking for another fish to live in.