Fish vs Pisces the Fishes

Fish and Pisces the Fishes both are nouns.

Word	Noun	Adjective	Verb	Adverb
Fish	Yes	No	No	No
Pisces the Fishes	Yes	No	No	No

As nouns, Fish and Pisces the Fishes are synonyms defined as:

Fish and Pisces the Fishes: the twelfth sign of the zodiac; the sun is in this sign from about February 19 to March 20

Other synonyms of Fish include Pisces.

Fish (noun)	Pisces the Fishes (noun)
the twelfth sign of the zodiac; the sun is in this sign from about February 19 to March 20	the twelfth sign of the zodiac; the sun is in this sign from about February 19 to March 20
(astrology) a person who is born while the sun is in Pisces