Preheat the broiler. Coat a broiler pan with cooking spray. In a medium bowl, combine the oil, garlic, cumin, salt, 1 tablespoon lime juice, and the fish and toss to coat. Let stand 15 minutes.
In a small bowl, combine the mango, bell pepper, jalapeño, cilantro, if desired, and the remaining 2 tablespoons lime juice. Set aside.
Wrap the tortillas in foil. Remove the fish from the marinade and place on the broiler pan. Broil until opaque, 3 to 6 minutes. Transfer to a plate and place the tortillas in the oven to warm slightly, 1 minute. Flake the fish.
Top the tortillas with equal amounts of lettuce, fish, and salsa.