Marinate the Fish:

1. In a small bowl, combine the lime juice, olive oil, chipotle powder, cumin, paprika and salt, and stir until smooth.
2. Pat the fish dry with paper towels. Rub on both sides with the lime-chipotle marinade. Transfer to a plate and cover tightly with plastic wrap, then transfer to the refrigerator to marinate for 15-20 minutes.

Prepare the Avocado-Yogurt Sauce:

1. While the fish marinates, prepare the avocado-yogurt sauce.
2. In the bowl of a food processor, combine the avocado, yogurt, lime juice, cilantro, salt and water, and process until smooth and creamy. Transfer to a serving bowl and cover tightly with plastic wrap. Refrigerate until ready to use.

Cook the Fish:

1. In a large heavy-bottomed skillet set over medium-high heat, saute the fish in the remaining 2 tbsp olive oil until opaque and flaky, about 4-5 minutes per side.
2. Remove from heat, and transfer to a platter. Using a fork, break up each filet into bite-sized chunks. Cover with foil, and set aside while you warm the tortillas.

Warm the Tortillas and Assemble the Tacos:

1. In a heavy-bottomed skillet set over medium heat, warm the tortillas one at a time for 20-30 seconds per side, or until soft and flexible. Wrap the finished tortillas with a clean dishcloth to keep warm between batches.
2. To assemble, place three tortillas on each plate and drizzle with avocado-yogurt sauce. Divide the fish evenly between the tortillas, then serve immediately with bowls of toppings so that everyone can top off their tacos as they please.