In a pitcher, mix the clementine and grapefruit juices. Set aside and keep chilled.
Place 4 to 5 ice cubes plus a few orange slices in each of 8 highball glasses. Divide fruit juice mixture among the 8 glasses, followed by 1/4 cup (60 mL) carbonated spring water for each.
To finish, add 1 tbsp (15 mL) of the grenadine syrup to each glass. To allow the grenadine to settle to the bottom of the drink, do not stir. Garnish with a strawberry or peach slice, if desired. Serve immediately.