Ichthyophthirius multifiliis
Ich is a parasite, it can only survive by living on the skin of the fish, this is called being "obligate".  The parasite moves by a fish carrying it, other animals, or even humans. The life cycle has three stages: tomont, theront, and trophont. The tomont stage is the adult stage of the parasite.  When this adult tomont leaves the fish, it falls to the bottom of the pond. Once the tomont falls to the bottom, it forms a thin cyst wall. Within that cyst wall the tomont begins to divide, creating thousands of tomites. Once all the tomites are made, they are released into the water. Once this occurs, the tomites then develop into an elongated form and are then known as a theront. Theronts are the developmental stage that then can swim to the fish and burrow itself into the epithelial skin. They do this by using a penetrating gland. What makes them an obligate parasite is that if they are not able to penetrate the fish’s skin they will die, so to survive, they need to get into the fish. Once they are in the fish’s skin they are then known as trophonts. The trophonts are protected from the chemical treatment in aquariums or ponds by the fish’s skin, and therefore it is difficult to get rid of them once the fish is infected. The trophonts then mature by feeding on the fish and become adults. The life cycle continues when the adults fall off again. The only stage that can be killed in the water is the tomonts and the theronts.