Place basil, jalapeño slice(s) and grapefruit wedge in a cocktail shaker, and gently muddle to release oils and flavors. Add lime juice, simple syrup, tequila, grapefruit juice and ice. Cap the shaker, and shake until well chilled. Strain into a tumbler filled with ice, then top off with soda water. Stir to combine. Garnish with a grapefruit wedge.