In a bowl, combine raw fish and lime juice. Cover and refrigerate until fish is no longer pink when broken apart, about 6 hours.
Drain in a colander, pressing gently to remove excess lime juice; return to bowl and refrigerate until ready to serve. In separate bowl, combine tomato, olives, cilantro and chiles. Place onion in a strainer; rinse under cold water; drain and add to tomato mixture. Just before serving, mix drained fish with tomato mixture and the olive oil. Taste and season with salt and sugar (if needed). Stir in diced avocado.
To serve, spoon a heaping tablespoon of the fish mixture on each tortilla chip; top each with a cilantro leaf, if desired. Arrange tostaditas on serving plates or a platter.