Shred cabbage and chop onion and cilantro. Set aside. Mince garlic.
Place the fish fillets in a dish and squeeze lime juice from half a lime over fish. Sprinkle fillets with minced garlic, cumin, chili powder, black pepper and olive oil. Turn filets to coat with marinade and refrigerate for 15 to 30 minutes.
Make salsa blanca by combining cucumber, mayonnaise, sour cream, milk and juice of half a lime. Stir to combine and place in refrigerator to chill.
Turn oven on to broil. Cover broiler pan with foil. Broil fish until the flesh turns opaque and white, and the fish flakes easily, about 10 minutes. Remove from oven, cool slightly and flake fish into large pieces.
While the fish is cooking, heat the corn tortillas, one by one, in a pan, until soft and warm. To assemble tacos place a piece of fish on tortilla and top with salsa blanca, cabbage, red onion, cilantro and lime wedges.