Coat the bottom of a large stock pot in olive oil. Heat over medium heat until oil is hot and begins to shimmer. Add the onion, fennel, crushed red pepper, basil. Season with salt and pepper to taste. Saute until onion is translucent and fennel is soft. About 10 minutes.
Add seafood stock, wine, diced tomatoes, and tomato paste. Bring to a boil, and then reduce heat to low. Simmer for 30 - 45 minutes over low heat.
Stir clams and mussels into the broth. Simmer for 3-5 minutes, or until shells are just beginning to open.
Stir fish and shrimp into the broth. Simmer until shrimp are pink, the fish is just cooked through, and the clams and mussels have opened. About 5 minutes.
When selecting a fish to use in your cioppinio it's best to select a fish that has firmer texture such as halibut, salmon, or cod. Try stopping by the fish counter at your local grocery store and asking if they have any fish pieces that are too small to sell. You can get a nice assortment for a deal!