1. Pour the water into a medium-sized nonreactive saucepan. Add the zest from the grapefruit, orange, lemon, and lime. (You can remove it with a sharp vegetable peeler, in strips, or with a citrus zester.) Halve, then juice the citrus fruits and add the juice to the saucepan.
2. Add the lemongrass, citric acid, chinchona bark, allspice, cardamom, star anise, salt, and black peppercorns. Bring the mixture to a boil and reduce to a simmer. Cover with a lid, leaving it slightly askew, and let it simmer gently for 15 minutes. Remove from heat, uncover, and cool to room temperature.
3. Pour into a container, such as a large screw-top jar, and chill for 2 days in the refrigerator, shaking it gently a couple of times a day.
4. Strain liquid through a fine mesh strainer, preferably into a large measuring cup (which will make the next step easier). Discard the spices, lemongrass, bark, and citrus peels. Strain the mixture again, this time through a strainer lined with several layers of cheesecloth, muslin, or a coffee filter. (If using a coffee filter, it’ll remove most traces of the spice powder but it’ll take a bit of time, so be patient.)
5. Add the sugar syrup, then pour into clean bottles or screw-top jars and refrigerate until ready to use.
To use the tonic water: Pour off the tonic water, avoiding disturbing any bark and spice sediment that might settle into the bottom of the bottle or jar, then add an equal amount of sparkling water to obtain the quantity that you need. So to make 1 cup (250ml) of tonic water, you’ll use 1/2 cup (125ml) of the tonic water mixture, and 1/2 cup (125ml) sparkling water.