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 whisk. Add potato, salt, and pepper; bring to a boil. Reduce heat. Simmer, uncovered, 30 minutes; stir occasionally. Add fish and 1 cup milk; cook an additional 10 minutes or until fish is done. Serve with oyster crackers, if desired.