Place the fish fillets in a non-reactive shallow plate. Sprinkle 4 tablespoons of the fresh lime juice all over the fish. Sprinkle with the 1 teaspoon salt, black pepper and my sweet Mexican spice if desired. Cover fish and chill for at least 30 minutes. (This could be done several hours in advance.).
If desired, lightly fry the corn tortillas in a little bit of oil in a pan. Cook unti crisp, about 1 minute on each side. Set aside to drain.
In a small bowl combine the yogurt-lime sauce ingredients, the remaining tablespoon of fresh lime juice and the remaining 1/4 teaspoon of salt. Can be prepared several hours in advance.
If you fried the corn tortialls, wipe the oil from the pan and heat up a little bit more oil. When pan is hot, add the fish along with 1 tablespoon of fresh lime juice, cover and cook until opaque, 10-12 minutes. Check for doneness.
Once cooked, break up the fish fillets into pieces.
Just before serving the tostadas, pit, peel and slice the avocado. Drizzle on some lime juice to stop the avocado from browning.
For each serving, place a tortilla on a plate and divide the cabbage, flaked fish with juices, the yogurt-lime sauce, cilantro leaves, salsa and avocado slices. You know the rest: rice and beans on the side, mariachi music, Pacifico, lime wedges, etc.