The question does not make sense. The correct grammar is "a fish" not "an fish."
Whale-small fishies-plankton- algea?
Starfish are aquatic invertebrates but are oftenly called "Echinodermata" or the class Asteroidea. Happyanswers56: Starfish are aquatic invertabrates. so it's right.
A fish is one fish and fish fish is two fish.
I prefer fish as the plural form, but fishes is also used.