In a mixing bowl, prepare Jell-O according to package directions. Place bowl over an ice bath, stirring occasionally, chilling it just until it becomes the consistency of honey, about 30 minutes. (If the Jell-O is in a metal bowl or pot this process will go much faster.)
While the Jell-O is chilling, use fish-shaped cookie cutters to cut the sliced fruit into the shapes. Assemble them against the sides of clear glass cups, a clear glass trifle dish or a bowl lined with plastic wrap. Place in the freezer for a few minutes so the fruit is nice and secure on the sides.
When the Jell-O is at the right consistency, carefully pour it into the glasses or bowl then refrigerate until set.