Marinate the fish in the olive oil, lime juice, salt and pepper for 30 minutes. Prepare the dressing by combining the ranch dressing and sriracha sauce in a small bowl. Set aside. Preheat oven to 250 degrees, wrap tortillas in foil and bake for 5 to 10 minutes or until warm.
Preheat the barbecue to medium heat. Remove fish from marinade and place on the grill. Grill fish until cooked through, but not overdone. Build the taco with fish, cabbage, onion, guacamole and top with the sriracha ranch dressing, cilantro and a squeeze of fresh lime juice. Serves 3-4.