Source: https://en.wikipedia.org/wiki/Royal_tetra
The royal tetra (Inpaichthys kerri) is a species of characin endemic to Brazil, where it is found in tributaries of the Aripuanã River. It is the sole member of its genus.
It is kept as an aquarium fish where it is sold under the additional common names blue emperor tetra, purple emperor tetra, black emperor tetra, or regal tetra (not to be confused with Nematobrycon palmeri with similar appearance and common names). I. kerri requires similar-sized schooling fish that are not overly aggressive, as this species is prone to diseases when stressed in captivity.