hot-water heater vs hot-water tank

hot-water heater and hot-water tank both are nouns.

Word	Noun	Adjective	Verb	Adverb
hot-water heater	Yes	No	No	No
hot-water tank	Yes	No	No	No

As nouns, hot-water heater and hot-water tank are synonyms defined as:

hot-water heater and hot-water tank: a heater and storage tank to supply heated water

Other synonyms of hot-water heater include water heater.

hot-water heater (noun)	hot-water tank (noun)
a heater and storage tank to supply heated water	a heater and storage tank to supply heated water