Good question
Deut 4:15-19
15 Take ye therefore good heed unto yourselves; for ye saw no manner of similitude on the day that the LORD spake unto you in Horeb out of the midst of the fire:
16 Lest ye corrupt yourselves, and make you a graven image, the similitude of any figure, the likeness of male or female,
17 The likeness of any beast that is on the earth, the likeness of any winged fowl that flieth in the air,
18 The likeness of any thing that creepeth on the ground, the likeness of any fish that is in the waters beneath the earth:
19 And lest thou lift up thine eyes unto heaven, and when thou seest the sun, and the moon, and the stars, even all the host of heaven, shouldest be driven to worship them, and serve them, which the LORD thy God hath divided unto all nations under the whole heaven.(KJV)
The fish, as a pair of fishes, is part of the Chaldean (Babylonian) zodiac, as one of the signs of astral worship with the Sun at the center of the zodiac. It therefore formed part of, and was a sign of Sun-worship.
The son of the Syrian goddess Atargatis was known as Ichthus too. Another Fish-deity was the Babylonian Ea, who became known amongst the Greeks as Oannes, similar to the Fish-deity of the Philistines, Dagon, half man and half fish.
The Fish is also associated with the sun...the god of the sun...as a fish," also being an Egyptian phallic emblem, as well as a sign of fecundity, or the female generative organ, or of female goddesses.