Heat olive oil in a large pot over medium heat. Add the chopped onion, minced garlic, diced celery, and diced bell peppers. Sauté for about 6 minutes until the vegetables are soft.
Pour in the fish stock and crushed tomatoes. Season with salt, pepper, and red pepper flakes. Bring to a simmer, cover, and let cook for 20 minutes to meld the flavors.
Add the clams and cook for about 4 minutes until most are open. Add the mussels and cook for 3 more minutes. Then add the shrimp, white fish, and crab legs or scampi to the pot.