In a small bowl or measuring cup combine granulated sugar (3 tbs) with 2 tbs warm water. Stir until the sugar is fully dissolved to make a very small simple syrup. This concentrates sweetness so it blends into cold carbonated water without grittiness.

Combine citrus

Add fresh lemon juice (1 tbs) and fresh lime juice (2 tbs) to the simple syrup and stir to combine. Taste: you should have a bright, slightly tart-sweet citrus concentrate.

Build the soda

In a large chilled pitcher, pour 4 cups chilled carbonated water. Slowly stir in the citrus-syrup mixture to avoid losing too much carbonation. Add 1 cup additional chilled club soda (or extra carbonated water) to boost fizz if desired. Stir gently 3–4 times.

Aromatics & Finish

Add the lemon zest (1 tsp) and lime zest (1 tsp) to the pitcher and stir gently once to release oils into the soda. If using, clap the mint leaves between your palms to release aroma and add to the pitcher.

Fill glasses with ice (about 1 cup per pitcher serving as indicated below). Pour the prepared soda over the ice. Garnish each glass with an optional mint sprig and a thin lemon or lime wheel.

Serve immediately to preserve carbonation. If storing, keep chilled and covered; use within a few hours for best fizz and flavor.