Does anyone know when the Christian fish symbol changed into the currently more well-known cross symbol?

Christians used to use the fish symbol (like the ones some people put on their cars) to represent their religion. This was based on the culture of fishing in Galilee, as well as, the Greek spelling for the word fish uses the same letters that represent "Jesus Christ, Son of God, Savior." Now, the cross symbol has become the most widespread symbol of Chrisianity, replacing the fish. Does anyone know when (possibly around what year) and why this happened (in a historical context)? Thanks so much everyone! I appreciate any explanations you have.