Here’s what I know:

A mainstream researcher in the history of languages would say that this Proto Indo European word is completely unrelated to the Christian fish symbol. When Christianity was born, the words for ‘fish’ and ’spirit’ were about 6,000 years separated; and besides, Jesus and his followers spoke Aramaic, an unrelated language. I tend to agree that it’s extremely unlikely that there’s a historical connection here. But that doesn’t mean that there isn’t a deeper spiritual connection between ’spirit’ and ‘fish’ — a connection that was picked up on by both the Christians and the ancient Indo Europeans.

As far as I know, Christianity is associated with the fish particularly because of what you mention — that some of Jesus’s followers were fisherman, and there is a famous scene in Matthew 4:19, when Jesus encounters the fishermen Simon, Peter, and Andrew, and says, “Come, follow me, and I will make you fishers of men.” They follow him; and immediately afterwards he meets two other fishers, James and John, who follow him as well.

I have also heard it said that Christianity’s fish symbolism is appropriate for the Age of Pisces, which began right around Jesus’s time and is ending now.