“it is a Halacha not to eat fish & meat together”
being in the catering business for many years i am fortunate to have a mashgiach working for me that takes his job very serious in which when working at Shabbos Simchas that on Friday nights the fish can stay on the table until the main course versus Shabbos day EVERY SINGLE piece of fish must be off the table before the next course of Liver-being meat-is served. even though there may be many empty seats still or people who have not arrived yet.
most people today will sadly break this halacha & you will see fish & meat together on the table. this is not a minhag but a halacha in shulchan aruch.