Rim a highball glass with salt by rubbing a lime wedge around the edge and dipping it into salt.
Fill the prepared glass with ice cubes.
In a cocktail shaker or separate mixing glass, combine the tequila, grapefruit juice, and lime juice.
Stir the mixture well to combine and chill.
Pour the tequila and juice mixture into the ice-filled highball glass.
Top the glass with sparkling mineral water.
Gently stir once to incorporate the carbonation without degassing.
Garnish with a fresh grapefruit wedge and serve immediately.