What is the smallest species of fish?

The smallest species of fish is the Stout Floater (Stygobromus hayi), measuring only about 8 millimeters (0.31 inches) in length. Found in subterranean aquifers in the United States, this tiny fish is an extreme example of adaptation to its environment. The Stout Floater has a reduced vision system, relying more on other senses to navigate its dark, underground habitat. Its small size helps it survive in confined spaces, and it thrives in the absence of predators and competition.