Dice fish and set aside. In a 4-quart Dutch oven, combine potatoes, water, onion, carrot, salt, and pepper. Cover and bring to a boil; reduce heat and simmer for 10 minutes. Stir in fish and cook covered for 10 more minutes. Stir in 1-1/2 cups milk and all the evaporated milk into the fish mixture. Blend remaining milk into the flour and stir into the fish mixture. Cook and stir until thickened and bubbly. Season to taste. Serve in bowls and top with pats of butter. Serves 8.