There are many versions of fish soup, each Italian region has its own. This is a classic version, very convenient to eat because the fish are served already filleted avoiding your guests the unpleasantness of finding bones or having to clean the fish on the plate.
You can also use other varieties of fish; you could ask your local fishmonger to indicate the freshest qualities available.
Remember that a good fish soup should be made up of a good variety of fish and should include:
redfish, gurnard, or similar tasty soup fish
prawns, shrimps, or other crustaceans
mussels or other bivalve molluscs like clams
squid or other cephalopod molluscs like red squid, octopus, or cuttlefish
If buying fish that is already filleted, ask the fishmonger to give you the heads as these are essential to make a good fish stock.
You can also flavour the soup with other herbs, including fresh basil or thyme.