Was raised on Mexican food. A mouth full of flavor from chiles and spices that add depth to this dish. Take a trip to Mexico. These will take you there. Eating these reminds me of walking the streets of Mexico standing at a Taqueria and drinking a bottle of Jarritos Lime or Guava soda.
The fish is easiest to dice when it is partially frozen, this also makes for a much cleaner presentation. Diced the shrimp the same size as the tilapia.
Put the tilapia and shrimp in a medium bowl. Pour the lime juice over the fish mixture and mix gently to combine.
Cover with plastic wrap and chill in the refrigerator until the fish is white throughout, about 20 minutes.
Remove from the refrigerator and drain off the lime juice, gently squeezing the fish mixture with your hands. Discard the lime juice. Add the tomato, cucumber, onion, and cilantro. Season with salt and pepper, to taste. Add the clam-tomato juice and the hot sauce and chile, if using. (If using chile remove the seeds and ribs if want a little spice go with the jalapeno and dice fine). I prefer the serrano and leave in the seeds and the rib.
Spread the tostadas generously with the lime mayonnaise this is found in the mexican food aisle and top with the ceviche. Arrange the avocado slices on top of the ceviche and serve immediately with the lime wedges alongside.