Which freshwater fish is the smallest known vertebrate species by standard body length?

Paedocypris progenetica is the smallest known vertebrate species by standard body length.

This tiny cyprinid was described from peat swamps on the Indonesian islands of Sumatra and Bintan. Mature females can measure about 7.9 millimetres in standard length, the measurement from the snout to the base of the tail. Its minute size results from extreme miniaturization, a process in which adult animals retain unusually juvenile-looking features.

The species lives in acidic, dark, highly specialized peat-swamp waters. Such habitats are vulnerable to drainage, fire, agriculture, and other environmental changes. Claims about the “smallest fish” can vary depending on whether researchers compare standard length, total length, or mature individuals, but Paedocypris progenetica is widely cited for the smallest vertebrate distinction.