Now while pagan worshippers for thousands of years prior to this manifestation may have used a fish as a symbol of "fertility", and the Ancients the Cross (merely the same two bi-secting lines I used as a child to create the "symbol" of an airplane), what matters is the emblematic connotation for what has been the last two-thousand years of such symbols.
It is also interesting to note so powerful are the Christian "symbols", that forces antagonistic to their connotations use the subverted versions to psychologically "help" those who are diametrically opposed to such Christian belief (upside-down crosses, reverse-reading of Scripture, Darwin Fish, etc.)
Prior to and during. When seen objectively, this is not a bad thing. The Christians took the most beloved of symbols, with all positive connotations, as their own. Knowing that no religion grows in a vacuum (look at the parts of Islam founded on Christianity, and the parts of Christianity founded on Judaism for clear examples) it's not surprising.