Salt the rim of a tall Collins glass by slitting a lime wedge in the middle and running the slit around the rim of the glass.
Dip the moistened rim in a small plate of kosher salt and shake off the excess.
Combine the tequila, grapefruit juice, and agave syrup in a shaker with ice. Shake to combine and pour into the Collins glass. Top with the sparkling water and garnish with the grapefruit wheel.