Christians, why did our symbol change from a fish to a cross?

[Nobody you need to know.]

I'd actually prefer a fish, since it was biblically used by the prophets. I mean, sure, Jesus died on the cross for our sins and we should remember that, but why can't just commemorate it on Easter and have the fish as a symbol?

It represents the miracles that God and Jesus that happen for those who are willing to give Him a chance.

*sigh*, oh well.