tank vs hot-water tank

tank and hot-water tank both are nouns.

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

As nouns, hot-water tank is a hyponym of tank; that is, hot-water tank is a word with a more specific, narrower meaning than tank:

tank (noun)	hot-water tank (noun)
a large (usually metallic) vessel for holding gases or liquids	a heater and storage tank to supply heated water
an enclosed armored military vehicle; has a cannon and moves on caterpillar treads
a freight car that transports liquids or gases in bulk
a cell for violent prisoners
as much as a tank will hold

tank (verb)	hot-water tank (verb)
treat in a tank
consume excessive amounts of alcohol
store in a tank by causing (something) to flow into it