Place fish filet in a bowl, add lime zest, lime juice, salt, pepper, and chili powder. Stir to coat fish. Place fish on a greased baking sheet. Bake at 375 degrees for 10-15 minutes, until fish is done and flakes apart easily. While fish is baking, slice the lettuce. Mix the sour cream and cilantro. Slice scallions.
When fish is done, flake into pieces.
In a taco shell, place a few pieces of fish, lettuce, salsa, sour cream mixture, and scallions.
**NOTE** If you wish, you could also top your tacos with avocados, tomatoes, black olives, black beans or your favorite toppings!