If using fresh clams, combine them in a large pot with 3/4 cup water. Cover, and bring to a boil over medium-high heat. Cook until the clams open, 3 to 4 minutes. Transfer clams to a bowl, discard any that are broken or unopened, and strain liquid into another bowl through a fine-meshed strainer lined with cheesecloth. Remove meat from shells, and chop into bite-size pieces. Reserve clams and liquid separately.
Combine butter and oil in a large soup pot, and place over medium heat. Add carrots, onion, celery and garlic, and sauté until the onions are translucent, about 3 minutes. Add zucchini, potatoes and parsley. Cover, and cook until vegetables are tender, about 10 minutes. Stir in wine and 2 cups of reserved or bottled clam juice. Add tomatoes and 2 1/2 cups water. Bring to a boil, then reduce heat to medium-low.
Add shrimp, fish and clams. If additional liquid is needed, add more clam juice or water. Cook for 10 minutes, until fish is flaky. Season with salt and pepper to taste. Sprinkle with parsley, and serve immediately.