From FyshyWyky

The Fysh were originally those undergraduates of the Department of Computer Science at The University of Warwick that frequented one particular computer laboratory there. This laboratory had one side of floor-to-ceiling windows, hence it looked somewhat like a fish tank. However, not letting logic get in the way, the laboratory became known as the fish bowl, or with the affectation of a y instead if i The Fysh Bowl. Thus the denizens of said laboratory called themselves The Fysh.

This term was in common use by them, and in reference to them by others, by at least early 1991.