DALLAS, Oct. 23 — What could be more peaceful, more restful or more relaxing than dropping a line into a quiet Texas lake and trying to hook a fish that is on Prozac? According to a study by a Baylor University toxicologist, fluoxetine — the active ingredient in the antidepressant Prozac — is making its way to a lake in the Dallas area and into the tissue of the freshwater blue gill fish.