Feel free to salt the rim of your glass-- pour a thin layer of salt on a small plate. Rub the wedge of a lime around the rim, then dip the glass into the salt so that it sticks. Fill the glass with ice.
In a small cocktail shaker or a large glass, add tequila, fresh lime juice, grapefruit juice, jalapeño and ice. Tightly close and shake vigorously for 20-30 seconds, then pour over ice in your margarita glass and finish it off with carbonated water. Add two thin slices of jalapeño and/or a slice of lime for garnish and enjoy! :)