Heat oven to 425°F. Line a baking pan-even if its nonstick-with aluminum foil and place fish on it. (Trust us, cleanup will be a lot easier if you use the foil.) Season both sides of the fillets with salt, pepper, and the juice from 1/2 lime. Bake for 10 to 12 minutes; you'll know fish is done when it flakes easily with a fork.
Shortly after you put the fish in the oven, start heating up the taco shells-check the box for exact instructions. Fill the shells with fish, lettuce, salsa, and avocado. Squeeze the rest of the lime over tacos right before serving for a quick flavor boost.