Heat olive oil and butter in a large pot over medium heat. Add celery and onion and cook about 3 minutes. Add tomatoes with juices and garlic. Simmer for 10 minutes.

Add chicken broth, water, and white wine to the pot with the vegetables and bring to a boil, about 5 minutes. Add parsley and saffron, and season with salt and pepper. Simmer broth 15 minutes more. Add fish to the soup.

Place a steamer insert into a saucepan and fill with water to just below the bottom of the steamer. Bring water to a boil, about 5 minutes. Add mussels and clams, cover, and steam until shells open, 5 to 10 minutes.

Transfer steamed mussels and clams and the steaming liquid to the pot with the soup. Simmer soup 10 minutes more.