Pour 2 cups of water into a medium-size, nonreactive saucepan. Using a vegetable peeler, add zest from the grapefruit, orange, lemon, and lime. Halve and juice the citrus fruits; add juice to saucepan. Add lemongrass, citric acid, dandelion root, all spices, and salt to the saucepan. Bring mixture to a boil. Cover, leaving lid slightly askew, and simmer for 30 minutes. Set aside to cool completely.

Pour into a large, nonreactive container, such as a Mason jar, and chill for 2 days in the refrigerator, shaking it gently a couple of times a day.

Strain through a cheesecloth-lined strainer and discard peel and spices. While mixture is straining, make a simple syrup. Combine sugar and 1 cup water in a saucepan; heat and stir until sugar completely dissolves. Remove from heat and let cool. Add simple syrup to strained tonic. Store in the refrigerator.