Tonic water is not the same as club soda, although both products are carbonated. While tonic water and club soda look much the same, the taste is different and the two products are not interchangeable in mixed drinks.
Club soda is carbonated water that contains added salt. Other flavorings also are added to some club soda brands. Tonic water, which got its start as a medicinal product, has more of a bitter flavor. The flavors of club soda and tonic water are used to balance out the taste of the respective liquors they are paired with, which means that each of the mixers serves a distinct role.