In a small pitcher or heatproof cup, combine the purified water, citric acid, and sodium citrate (if using). Stir until dissolved. Add the liquid monk fruit or erythritol-based sweetener and stir well to incorporate. This concentrated solution helps the flavor distribute evenly in carbonated water.

Combine with carbonated water

Place the chilled carbonated water in a large chilled bottle or pitcher. Add the sweet-acid solution to the carbonated water gently. Pour slowly down the side of the container to preserve fizz. Taste and adjust: add up to 1 tbs additional sweetener if you prefer sweeter, or an extra 1/4 tsp citric acid for brighter acidity.

Flavor Variants (instructions for preparing each flavor concentrate; add to 1 liter base seltzer)

Lime

In a small bowl, combine fresh lime juice and lime zest. Strain briefly to remove pulp if desired. Stir the lime mixture into the prepared base seltzer. For a brighter profile, lightly muddle 1 extra teaspoon of zest in the bottom of the serving glass and then pour seltzer over it.

Raspberry

Place raspberries and 2 tbsp water in a fine mesh sieve or blender. Mash or blend until smooth, then press through a fine-mesh sieve to remove seeds, yielding about 2 tbsp raspberry purée. Add 1–2 tbsp purée to the base seltzer and gently stir. Adjust to taste — more purée for a stronger berry flavor.

Grapefruit

Combine fresh grapefruit juice and zest. Because grapefruit is more aromatic, start by adding 3 tbsp (about 1.5 oz) to the base seltzer and taste. Increase up to 1/4 cup if more pronounced flavor is desired. Stir gently to combine.

Cucumber-Mint

Place chopped cucumber and mint leaves in a blender with 1–2 tbsp of water and pulse briefly to release juices. Strain through a fine mesh sieve to collect a clear cucumber-mint concentrate (about 2 tbsp). Add 1–2 tbsp concentrate to the base seltzer and stir gently. For a stronger aroma, lightly clap mint between your palms and add as a garnish.

Peach

Blend the chopped peach into a smooth purée. If using frozen, thaw slightly first. Press through a sieve if you prefer seedless clarity. Stir 1–2 tbsp peach purée and 1 tsp vanilla extract (optional) into the base seltzer. Adjust quantity up to 3 tbsp for a more pronounced peach note.

Ginger-Lemon

Grate fresh ginger and collect juice by pressing grated ginger in a sieve or cheesecloth; combine with fresh lemon juice. Start by adding about 1–2 tsp of the ginger-lemon concentrate to the base seltzer because ginger is potent. Stir and taste, then add more in 1/2 tsp increments if needed.

Optional Garnishes and Serving

Fill a serving glass with ice if desired. Pour the flavored seltzer over the ice. Garnish with herb sprigs, thin citrus slices, or a few whole raspberries/peach slices as appropriate to the flavor. Serve immediately to preserve carbonation.

Storage: Keep unopened flavored seltzer chilled and sealed; consume within 24–48 hours for best fizz and fresh flavor. If you need to store longer, keep the base carbonated water separate and add flavor concentrate just before serving to preserve effervescence.