1. Remove the zest from the orange and lemon with a potato peeler. Place the zest, and artificial sweetener equivalent to 3 tablespoons of sugar, in a bowl. Pour the boiling water into the bowl and leave until cold.
2. Strain the zest from the liquid, pour into a blender, add the chilled soda water and ice cream. Blend for 1 minute until very frothy.
3. Pour the ice cream soda into four glasses and decorate each glass with a slice of orange or lemon. Serve immediately.