In a medium bowl, combine all ingredients for the marinade with the fish. Marinate for at least 4 hours in the refrigerator.
Making tostadas at home is very simple. Preheat oven to 425°F. Brush the corn tortillas with a little oil. Place them on a rack and bake until golden brown. Season with salt when they come out and let them cool to room temperature.
Place one tostada on six plates. Divide marinated fish among them.