The japanese style is to use half club soda half tonic. It's supposed to bring out the taste of gin better, i am trying it and it's pretty good. They call it a Gin and Sonic (soda water + tonic = sonic)https://www.liquor.com/recipes/gin-sonic/
The Gin Sonic (soda + tonic = “sonic”) has been popular in Japan for years, but only began to find a footing in the United States in the late 2010s. Now, many bartenders around the country are making the light, refreshing cocktail in their bars, helping to spread its charms to drinkers more accustomed to Vodka Tonics and G&Ts.
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The name suggests something big and powerful. But at its chiseled core, the Gin Sonic is a somewhat healthier version of the Gin & Tonic, comprising 1 1/2 ounces of a quality gin with 2 1/4 ounces each of club soda and tonic water. It not only reduces the amount of sugar and some of quinine’s inherent bitterness but also allows for the botanical flavors to pop.