In a pitcher, mix together the tonic water and 2 cups regular water and pour into fun-shaped ice cube trays. Place the trays into the freezer and freeze for 24 hours.
Divide the grape juice, vodka and lime juice between 2 highball glasses. Add the black light ice cubes to the top (do right before serving because the ice melts quickly). Top with the remaining tonic and serve. Black light optional.