Clean and scale (if necessary) all the seafood. Check the calamari for any cartilege and cut into 1-inch pieces. Pour 1 tablespoon olive oil in a medium-sized saucepan with a lid. Heat over a medium-high flame until hot but not smoking. Add the mussels and cover, steaming until the mussels open, about 5 minutes. Remove the mussels, reserving the juices left behind in the pan.
In a 12 to 14-inch saute pan, add the remaining oil and heat over a hot flame until hot but not smoking. Add the sliced garlic, parsley, celery, onion, and chili pepper. Cook until the garlic softens and turns a light golden brown, about 5 minutes. Add the wine and let it boil and evaporate. Add the chopped tomatoes, salt to taste and continue cooking, stirring occasionally, for about 5 minutes. Add 2 cups water, reserved mussel juices and the remaining fish (as whole) and bring to a boil. Let the mixture cook at a bare boil, uncovered, until the fish is opaque and flaky, about 20 minutes. Remove the fish from the pan place in a wide flat soup tureen and set aside in a warm place.
Bring the liquid to a boil over high heat, add the shrimp then lower the heat and let simmer until the prawns are opaque, about 3 minutes. Add the calamari, and the shucked mussels and let simmer until all the calamari is just cooked through, about 2 minutes more.
Cut the remaining garlic clove in half, and use the cut end to rub the toasted bread, imparting a garlic flavor. Discard the garlic clove. Serve the soup hot in warmed soup bowls with the garlic-rubbed toast on the side.