Sauté onions, garlic, celery, fennel and parsley in olive oil in a sturdy stockpot over a medium flame until tender. Add tomatoes and paste, clam juice and crushed red pepper. Bring to a steady boil. Reduce heat and simmer for an hour or so. Add water to maintain volume of liquid.
Hold crabs firmly to remove the bodies from shell. Split bodies in half and crack legs. Add cracked crab, crab fat, clams, scallops and fish to the sauce. Simmer for a quarter of an hour at least. Add in calamari rings and prawns cooked in a separate pan for the last five minutes of cooking. When all the clams have opened, the stew is ready. Serve with fresh green salad and sourdough bread and a hearty red wine.