Preheat oven to 180 degrees C. Lightly grease a medium baking dish.
In a medium bowl, mix the cayenne pepper, garlic powder, dill weed, thyme, salt and pepper.
Brush the fish fillets with the margarine and press into the spice mixture to coat.
Arrange fillets in the prepared baking dish, and bake 20 minutes in the preheated oven. Top with the partially drained diced tomatoes and continue baking 10 minutes, or until fish is easily flaked with a fork.