In today’s world, is not eating meat together with fish a halacha, a minhag, or other. If a minhag, does it apply to particular groups: Ashkenazi, Sefardi, Chasidic. Any other information you can relate regarding this practice would be appreciated.
Answer:
Not eating fish with meat is a halacha, mentioned in the Talmud and codified in Shulchan Aruch. The reason for this prohibition is due to danger involved to one who eats both together. There are authorities who maintain that there is room for some leniency as we no longer perceive a danger from this practice. Nevertheless, all communities across the spectrum still refrain from the this practice.
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