The fish is a Christian symbol
The swastika used to be a good luck symbol, before Hitler & Co.
The rainbow used to be a symbol of mercy (God's promise to never again destroy mankind with a Flood), before gays & co.
Touching the tip of your pointer finger to the tip of your thumb used to be a symbol for "okay", until 2017 when it became a symbol for white supremacy.
Identical symbols carry different meanings in different times and cultures, and just as words can have multiple meanings, so too symbols can have multiple meanings, even simultaneously.
Since the time of the first century Roman Christians in the catacombs, the fish has been a symbol of Christianity in much of Christendom. That does not mean it has not had and does not have other meanings.
https://creation.com/jesus-fish-darwin-fish