In a large pot, combine the soup mix, bay leaf and fish stock. Cover and bring to a simmer.
Reduce the heat to medium-low. Cover and simmer about 30 minutes.
Add the clams and mussels to the cooking liquid. Cover and cook until the clams and mussels begin to open, about 5 minutes.
Add the shrimp and fish. Simmer gently until the fish and shrimp 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).
Season the soup with red pepper flakes. (Optional).
Ladle into bowls and serve.