Until not so long ago I did not even know that making your own tonic water was possible. With all the brands of Tonic water available these days you might be happy to stick to the shop-bought stuff. Give this a try though. You might just be converted! Home-made tonic water is slightly bitter, but much less sweet, and also lower in kilojoules. When used to make a G&T they allow the flavor profile of the gin to come through.
Be sure to read my post The Gin Revolution for more info on the history of tonic water.
The ingredients in the photo are as follows: (clockwise from left to right) lemongrass, allspice, cardamom pods and chinchona bark.