Salt the rim of the glass by rubbing a grapefruit wedge around the edge and dipping it onto a small plate of salt.

Mix the tequila, grapefruit juice, sparkling water, lime juice, and agave nectar into the glass. Fill the remainder of the glass with ice. Adjust sweetness to taste. Garnish with grapefruit wedge.

To make concentrate: Mix all but club soda, about 30 oz. To make drink: to 5 oz concentrate, add 2 oz soda.