The fish symbol is representative of the "Age of Pisces" which began 1-AD and shall end 2150BC, prior to 1-AD we were in the "Age of Aries"(1-BC to 2150BC), this is why you see alot of Rams and Ram Horns in the Old Testament(Jews to this day still use Rams Horns), before the Age of Aries was the Age of Taurus in which the dominant Idols were represented by a Bull~ read Exodus 32 where Moses forbids the people to worship the "Golden Calf" which was considered paganism as the Age of Aries overtook Taurus.
With that said let me explain what an AGE is. In astronomy, axial precession is a gravity-induced, slow and continuous change in the orientation of an astronomical body's rotational axis. In particular, it refers to the gradual shift in the orientation of Earth's axis of rotation, which, like a wobbling top, traces out a pair of cones joined at their apices in a cycle of approximately 26,000 years, which our ancient ancestors divided up into quadrants that correlate to the Zodiac(12 astrological signs), if you divide 26,000years by 12, you get 2,166years(but it rounds out to 2,150 which is an "AGE").

After the Age of Pisces will come the "Age of Aquarius", read Luke 22:10(And he said unto them, Behold, when ye are entered into the city, there shall a man meet you, bearing a pitcher of water; follow him into the house where he entereth in.)- Jesus is referring to the Age of Aquarius.
The Bible(especially King James) is filled with bad translations and misinterpretations.. for instance, Matthew 28:20-"Teaching them to observe all things whatsoever I have commanded you: and, lo, I am with you always, even unto the end of the WORLD" ~ They say "World", however in King James version World is a mistranslation, the actual word being used is AEON, which translate to AGE!!! Hello!! Jesus says I will be with you til the end of this AGE!!!Which is true! ...but no end of the world like they tried to make it! Instead till we pass into the AGE of AQUARIUS!