Place clams & mussels in a sink with cold water. Scrub the mussels to remove any grit and cut away the "beards" with a sharp knife.
Chop the onion and parsley. Mince the garlic, carrots and celery.
In a large pot with a tightly fitting lid, heat the olive oil and add the chopped/minced vegetables. Cook over low/medium heat until soft but not browned. (5-6 minutes stirring intermittently)
Add the tomatoes, tomato puree, wine, clam juice, vinegar, herbs and red pepper flakes. Bring to a boil, then lower heat and simmer uncovered for 30-40 minutes. Then place in another container. (This can be done ahead of time, cooled and refrigerated but be sure to bring back to a simmer before adding to the fish when finishing the soup.)
While the soup is simmering, prepare the fish:
Cut halibut into serving size pieces.
Tear crabmeat or lobster into bite size pieces.
Throw away any mussels or clams that float to the surface in the sink, drain in colander.
Place the fish in the large pot, with the clams and mussels on the top. Pour over the soup that you've brought back to a simmer. Cover tightly and cook over low/medium heat for 20-25 minutes. Remove lid to see if all the clams and mussels have opened--if not, cook for another 5 minutes or until they open. It is unlikely that you'll need salt as the seawater released from the clams and mussels usually adds all the natural saltiness necessary.
Serve in large heated bowls with a good crusty bread alongside and a simple salad. Red wine goes nicely with this but white is fine as well.