From Drs Faoster and Smith Vet staff...Cut and paste
Why do fish get Cryptocaryon?
Cryptocaryon is a parasite, and like most parasites it is very prevalent in the environment of the species it normally infects. Therefore, most wild fish are exposed to low levels of this parasite fairly frequently and are able to effectively fight off the infection without becoming seriously ill. What happens in an aquarium fish, however, is very different than what happens in a wild fish. In the wild, the number of free-floating Cryptocaryon per 100 gallons of water is extremely small. Whereas in a home aquarium with a relatively small volume of water and a concentrated population of fish, the number of Cryptocaryon has the opportunity to explode into a number hundreds of times higher than what would ever be experienced in the wild. The other thing that happens in a home aquarium is that the level of stress has the potential to be much more severe than what is found in the wild.
Almost all marine aquarium fish are wild caught, and in a period of several days their lives change dramatically. They go from living on the reef to being collected, handled, shipped, and re-handled repeatedly. When those fish finally enter the home aquarium, they are then subject to yet another change in water parameters, diet, temperature, and environment and may even be subject to aggression from existing tank mates. To say the least, these fish are severely stressed to the point that their immune system may not be functioning properly, making them very susceptible to infections and parasites like Cryptocaryon. In an existing tank in which the fish are healthy, the introduction of a new fish or a decrease in water quality or temperature fluctuations may stress the fish to the point that the Cryptocaryon protozoans that were present but not creating problems will then rapidly cause a more serious infection.