tara, ich is an infectious parasite. Thus, it is caused by a fish coming into contact with the parasite. It is not caused by stress. However, a stressed fish can certainly become more symptomatic of disease because, like all things, living creatures are more susceptible to disease when stressed.
The best treatment for ich is hyposalinity. It is safe and very effective. The procedure is explained in the FAQ thread stickied to the top of this forum.