The two fish appear to be related to the astrological sign of Pisces.
In Aion, Jung studied the astrological symbolism of the precession of the equinoxes between the Ages of Pisces and Aquarius, as noted above, and the relation of this to the figures of Christ and the Antichrist.
He wrote: “the designation of Christ as the one fish identifies him in the astrological interpretation with the first vertical one. Christ is followed at the end of time by the Antichrist.
The beginning of the enantiodromia must fall, logically, between the two fishes.
We have seen that this is so.
The time of the Renaissance begins in the immediate vicinity of the second fish, and with it comes that spirit which culminates in the modern age” (§ 149 ).
At a psychological level, he noted, “If we see the traditional figure of Christ as a parallel to the psychic manifestation of the self, then the Antichrist would correspond to the shadow of the self, namely the dark half of human totality” (§ 76) .
He added that symbolically, “the coming of the Antichrist is not just a prophetic prediction- but an unrelenting psychological law ….
The ideal of spiritually striving for the heights was doomed to clash with the materialistic earth-bound passion to conquer matter and master the world.
This change became visible at the time of the ‘Renaissance'” (§§ 77- 78) . ~The Black Books, Vol. VII, Page 215, fn 179