In the interest of time and ease of preparation I suggest using frozen or canned seafood. We like frozen. The shells still being on the clams and mussels help the look and taste. But, it's even a little faster and still tastes great with canned seafood.
Put a pot of water on to boil and add the salt. Put a large Sauté pan on at medium high and add the marinara sauce, the lemon juice (and the rinds) and the pepper flakes. When the pasta water comes to a boil add the pasta and mix it right away. It should be done al dente at about the same time as your Zuppa di Pesce
Add the clams, mussels and calamari to the skillet. Bring it up to a fast simmer and lower the heat just enough to maintain the simmer. Simmer for about five minutes stirring a few times as it cooks.
When the calamari is soft add the shrimp and stir them into the mix. When the shrimp are opaque serve up your Zuppa di Pesce with the pasta. Garnish with a little chopped fresh basil or parsley and some grated parmesan cheese.