aquarium vs fish tank

aquarium and fish tank both are nouns.

Word	Noun	Adjective	Verb	Adverb
aquarium	Yes	No	No	No
fish tank	Yes	No	No	No

As nouns, aquarium and fish tank are synonyms defined as:

aquarium and fish tank: a tank or pool or bowl filled with water for keeping live fish and underwater animals

Other synonyms of aquarium include marine museum.

aquarium (noun)	fish tank (noun)
a tank or pool or bowl filled with water for keeping live fish and underwater animals	a tank or pool or bowl filled with water for keeping live fish and underwater animals