Is it permissible for the bride and groom to jump and spin around a fish at a wedding party, as a courtesy to people, while not believing in any benefit or harm from it, and to avoid shirk?

Amulets are a form of shirk (definition) (polytheism), and a fish hung up to bring benefit or ward off harm is considered among amulets. If the belief concerning the fish is that it affects benefit or harm independently alongside Allah, then this is major shirk. However, if the belief is that it does not affect independently, then it is minor shirk. One should hasten to change this evil if possible; otherwise, one must keep away from it and warn people against it.