Early christian inscription ichthys (symbol on left)  carved with Greek letters into marble in the ancient Greek ruins in Ephesus, Turkey.

The symbol of the fish is derived from the ichthyocentaurs, who aided Aphrodite when she was born from the sea.  The symbol of Pisces one of the earliest zodiac signs known, and has strong associations with early Christianity, and many of the disciples had been fishermen.  The sign of Pisces in the zodiac rules from February 19th to March 20th.

Zodiac symbol	fish

According to Wikipedia:

I had no idea we were in it, but the age of Pisces began c. 1 AD and will end c. 2150 AD.  With the story of the birth of Christ coinciding with this date, many Christian symbols for Christ use the astrological symbol for Pisces, the fishes. The figure Christ himself (is said) to bear many of the temperaments and personality traits of a Pisces (whaever these are…) and is thus considered an archetype of the Piscean.  Moreover, the twelve apostles were called the “fishers of men,” early Christians called themselves “little fishes,” and a code word for Jesus was the Greek word for fish, “Ikhthus.” With this, the start of the age, or the “Great Month of Pisces” is regarded as the beginning of the Christian religion. Saint Peter is recognized as the apostle of the Piscean sign.

The veracity of all this?    Well Wiki says so!  We have no pisceans in the family, and I’m only an occasional dabbler in astrology,  so I can’t verify the personality traits and so on  😉