Thaw fish, if frozen. In a small saucepan, stir together undrained tomatoes, onion, green chili peppers, garlic, and chili powder. Bring to boiling; reduce heat. Simmer, uncovered, for 5 minutes.
Meanwhile, measure thickness of fish. Grease rack of an unheated broiler pan. Place fish on rack, tucking under any thin portions. Brush fish with oil.
Broil 4 inches from heat until fish flakes easily with a fork. (Allow 4 to 6 minutes per 1/2-inch thickness of fish. If the fish is more than 1 inch thick, turn it over halfway through broiling.) Serve fish with tomato mixture. Makes 4 servings.