The Mer-Writers Thread

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What are some of the most common mermaid tropes? I'd say that unless a writer can do the common ideas better than they've already been done, they should be avoided. Unelss it's an exercise, but if you want to get anywhere as a writer, do the same old ideas keep selling after a while?

Some common mermaid themes I see around are:
Human discovers she's really a mermaid and must deal with the consequences.
Mermaid discovers she is really an important one/and or must save her kingdom.
Mermaid is matched to marry and wants a human instead.
Mer falls in love with a human and must choose between the mer world and theirs.

Which am I missing here?

And of course there are basics to the myths, which I don't think anyone need avoid if it's part of the story. Like vampires, weres, and other supernaturals, there are things that make a fantasy creature what it is. Mermaid myths are often associated with the full moon, tides, and being predatory, and I use those, but hopefully not in a way that's been done already.