Pour 1 cup of the 100% juice into a medium metal mixing bowl. Sprinkle on the gelatin and set aside to soften. Bring the remaining 3 cups juice to a boil. Pour the hot juice slowly into the softened gelatin, stirring until it is completely dissolved, 1 minute.

Refrigerate the gelatin until a thin film clings to the sides of the bowl when stirred, about 45 minutes.

Rinse a glass baking dish (8x6x2 inch) with cold water and pour the thickened gelatin into it. If the gelatin has begun to set, spread it out as evenly as possible.

One at a time, holding the fish upside down, push them into the gelatin so they look like they are swimming. Place them far enough apart to cut six blocks, each containing a fish.

Refrigerate the gelatin until very firm, 3 to 4 hours.

To unmold, hold the dish in a pan of hot water for 1 minute, or until the gelatin releases when you bounce the pan.

Invert the pan over a plate so the gelatin drops onto it. With a sharp knife, cut the gelatin into 6 blocks, including one fish in each. The blocks are best served the day they are made as the fish start to get gooey after 12 hours.