Season fish with seasoned salt and pepper. Heat oil in a large pan over medium-high heat and cook fish on both sides. Remove fish from pan and add corn kernels. Cook for 3 to 4 minutes. Break fish apart into large chunks.
For each tostada, place one tortilla on a plate. Top with one-fourth of the beans and corn. Top with 1/4 cup cheese, a few onion rings some fish and some salsa. Place the second tortilla over the salsa. Place 2 tomato slices on the tortilla. Add some more fish, then some salsa, some lettuce, more cheese and a tablespoon of sour cream.