Preheat the grill to medium-high heat. In a small bowl, mix together olive oil, chili powder, cumin, garlic powder, salt, and black pepper. Rub the spice mixture over the fish fillets.
Grill the fish for about 3-4 minutes on each side, or until it flakes easily with a fork. Remove from the grill and let it rest for a few minutes.
While the fish is resting, prepare the mango salsa by combining diced mango, red onion, jalapeño, cilantro, lime juice, and a pinch of salt in a bowl. Mix well and set aside.
Warm the corn tortillas on the grill for about 30 seconds on each side until they are pliable.
To assemble the tacos, place a few pieces of grilled fish on each tortilla, top with shredded cabbage, a few slices of avocado, and a generous spoonful of mango salsa. Squeeze fresh lime juice over the top before serving.