no no. You've got it all wrong.
The early Christians chose the fish to be secretive about being Christians when they were being officially persecuted by Rome. They chose the fish because Jesus made Peter his fisher of men (quote), and the rock upon which to build His church.
The anon didn't choose it. Christians circa AD 70 did to hide from the persecuting Romans who were rounding them up and feeding them to lions, or burning them in brass bulls, or crucifying them.
Early Christians had to remain ANONYMOUS, so it was a very good sign to be the meme that they could recognize each other by.
Its always interesting to me, what non-Christians think Christianity is, but I'm never surprised how you come out in droves at Easter (Resurrection) and Christmas (Christ's mass)