Preheat the oven to 225 degrees . Place the fish in a medium bowl and drizzle with half of the lime juice. Cover and refrigerate for 15 minutes.
Using your hands, squeeze the juice and flesh from the tomatillos, 1 at a time, into a medium bowl; discard the skins. Mash the tomatillos with a fork. Add the zucchini, the remaining lime juice, the cilantro and 2 teaspoons of the salt and toss.
Spread the tortillas on 2 parchment-lined baking sheets and warm in the oven for about 10 minutes.
In a small bowl, combine the chili powder, cayenne pepper and 1/2 teaspoon of the salt. Remove the fish from the lime juice, pat dry and cover with the spice rub. Heat the olive oil in a large nonstick skillet over medium heat and cook the fish, about 4 minutes on each side. Break the fish into bite-size pieces and season to taste with salt. Arrange the fish in the tortillas with some of the salsa and avocado slices.