Which freshwater fish is often cited as the world’s smallest fish species?

Paedocypris progenetica is often cited as the world’s smallest fish species.

This tiny cyprinid was described from acidic peat-swamp waters on the Indonesian islands of Sumatra and Bintan. Mature females can measure only about 7.9 millimetres in standard length, making the species one of the strongest candidates for the smallest fish title.

Its body is highly reduced and almost transparent, with a delicate skull and unusual anatomy. Like other tiny fish that compete for the record, its ranking depends on whether scientists compare minimum adult length, maximum adult length, or overall body mass. For that reason, references sometimes mention other minute gobies or dragonfish in the same discussion.

Paedocypris progenetica is not an aquarium variety or a juvenile of a larger species. It is a naturally miniature species adapted to specialized peat-swamp habitats, which are vulnerable to drainage, fire, and land-use change.