In a shallow dish combine the chili powder, cumin, salt and pepper. Dredge the fish in the spice rub.
In a large nonstick skillet heat the oil over medium heat. Add the fish and cook until fish flakes easily with a fork, about 3 minutes per side. Heat the beans and tortillas. Set aside.
In a small bowl mash the avocado. Stir in half the lime juice. Toss the shredded cabbage with remaining lime juice. Layer each tortilla with fish, black beans, cabbage, salsa, and a dollop of the avocado.