I am looking for a short story that was mentioned in the episode "The Unsettling Depths of Bird Intelligence" of the Stuff to Blow your mind pod cast. I could not make out the authors name clearly enough and any searches fail to combine the subject of the story to possible authors.
A (rather poor) transcript of the mention from ~35:30 Minutes into the episode:
If, in fact, [author name, possible Jorge Luis Borges] rainbow fish story is true, [...] it has to do with the fact that everything you see in the mirror, the mirror people, mirror creatures are merely repeating our actions, and they look like us, and they go through this silly mimicry because they lost a war ages ago, and part of the truce is they have to just mime everything we do. But one day they will rebel against us, and the first thing we will see in the mirror is the brilliant rainbow fish, you know, colours that we never see in this world, that will be the sign "oops, it's about to go nuts here".