Combine salt and lime zest on a small plate or shallow bowl. Run a grapefruit wedge along the rim of four highball glasses, press rim of each glass into salt and lime zest mixture to coat. Place grapefruit wedge into bottom of each glass.
Stir together lime and grapefruit juices. Add half of the jalapeño slices and all of the agave syrup in a pitcher filled halfway with ice. Stir in tequila.
Fill glasses with ice and divide remaining jalapeño slices among the glasses. Pour tequila mixture over and top with soda water.