Add the clams and mussels to the cooking liquid. Cover and cook until the clams and mussels begin to open, about 5 mins.
Add the prawns and fish. Simmer gently until the fish and prawns are just cooked through, and the clams are completely open, stirring gently, about 5 minutes longer (discard any clams and mussels that do not open). Remove bay leaf. Season the soup, to taste, with more salt and red pepper flakes.
Ladle the soup into bowls and serve.