1. Rinse fish and pat dry; cut fillets crosswise into 1-inch-wide strips. In a small bowl, mix cumin, chili powder, garlic powder, and salt. Rub fish all over with spice mixture. Rinse, core, and seed bell peppers; cut lengthwise into 1/4-inch-wide slices.
2. Pour 1 tablespoon oil into a 10- to 12-inch nonstick frying pan over medium-high heat. When hot, add onion and bell peppers and stir frequently until limp, 5 to 6 minutes. Transfer to a plate. In the same pan, heat remaining 1/2 tablespoon oil; add fish and cook, turning once, until fish is browned on both sides, about 4 minutes total.
3. Add mango juice and bring to a boil. Cook until fish is opaque but still moist-looking in the center of the thickest part (cut to test), 2 to 3 more minutes. Add bell pepper mixture and lime juice to fish and stir just until heated through. Serve with warm flour tortillas.