Bring 4 cups water to a boil in a large skillet. Add fish; cover, reduce heat, and simmer 7 minutes or until fish flakes easily when tested with a fork. Remove fish from skillet with a slotted spatula; set aside. Melt margarine in a saucepan over medium heat. Add carrot, celery, and onion; saute 2 minutes. Stir in flour. Gradually add 2 1/2 cups milk, stirring constantly with a wire whisk. Add potato, salt, and pepper; reduce heat, and cook, uncovered, 30 minutes, stirring occasionally. Add fish and remaining 1 cup milk; cook an additional 10 minutes or until thoroughly heated. Serve with unsalted oyster crackers, if desired. Yield: 4 servings (serving size: 1-1/2 cups). Calories 299 (21% from fat), protein 29.6g, Fat 6.9 g (Sat 1.5g, Mono 2.7g, Poly 2.1 g), Carb 28.3 g, Fiber 1.6g, Chol 53 mg, iron 1.3 mg, Sodium 395 mg, calcium 297 mg soup 63