poeciliid fish vs molly

poeciliid fish and molly both are nouns.

Word	Noun	Adjective	Verb	Adverb
poeciliid fish	Yes	No	No	No
molly	Yes	No	No	No

As nouns, molly is a hyponym of poeciliid fish; that is, molly is a word with a more specific, narrower meaning than poeciliid fish:

poeciliid fish: small usually brightly-colored viviparous surface-feeding fishes of fresh or brackish warm waters; often used in mosquito control
molly: popular aquarium fish

Other hyponyms of poeciliid fish include Gambusia affinis, mosquitofish, platy, Platypoecilus maculatus, mollie.

poeciliid fish (noun)	molly (noun)
small usually brightly-colored viviparous surface-feeding fishes of fresh or brackish warm waters; often used in mosquito control	popular aquarium fish