Every region of Italy with a coastline has a characteristic fish soup. This Roman version is spicy with garlic and hot chile. It contains a minimum of
liquid, so it is more like a stew than a soup. Use whatever fish varieties are available with the exception of strong-flavored oily fish, which would overwhelm the delicate shellfish.
In a large saucepan over medium heat, sauté the garlic and pepper flakes in the olive oil until the garlic is golden, about 2 minutes. Using a slotted spoon, remove the garlic and discard. Add the squid and cook and stir until opaque, about 2 minutes. Add the wine and simmer for 1 minute longer. Add the tomatoes, parsley, and salt and cook until the juices evaporate, about 10 minutes longer.
Add the water and bring to a simmer. Add the clams (discard any that do not close to the touch) and fish, cover, and cook until all the clams open and the fish is opaque throughout, about 5 minutes. Discard any clams that failed to open. Adjust the seasonings.
Place a bread slice in each warmed soup plate. Ladle the soup over the bread and serve.