Heat the oil in a large pot over medium heat. Add the celery, pepper, onion, and salt and saute until the onion is translucent, about 10 minutes.
Add the garlic and red pepper flakes, and saute for 2-3 minutes.
Stir in the tomato paste and cook for another 2 minutes.
Add tomatoes, water/stock and bay leaf. Cover and bring to a simmer. Reduce the heat to medium-low. Cover and simmer for about 15 minutes.
Add the clams and mussels to the pot and cover and cook until the clams and mussels begin to open, about 5 minutes.
Add the shrimp and fish, if using. Simmer gently until the fish and shrimp are cooked through.
Discard any clams and mussels that do not open)
Season the soup with more salt and red pepper flakes.
Ladle the soup into bowls and serve with crusty bread or garlic bread.