Is there an Inyan to specifically eat fish and meat at every Seuda on Yom Tov?
It is a mitzvah of Simchas Yom Tov to eat meat on Yom Tov both by night and by day, ideally during the seudah. See here:
While eating fish Yom Tov isn’t a mitzvah like meat, and the mitzvah of Oneg Yom Tov can be achieved through other goods one enjoys, it’s customarily served at a seudas mitzvah, and should be served on Yom Tov too, unless there are mitigating factors. Additionally, seforim mention many reasons why fish should be eaten on Shabbos – indeed the Alter Rebbe writes it should be eaten for Seuda Shlishis too – some of these reasons apply Yom Tov too. See attached.
See also references here: