Emperor Tetra (Nematobrycon palmeri)

Nematobrycon palmeri comes out of Colombia's Cauca and San Juan river drainages, and the purple-gold sheen running down its flank shifts noticeably depending on the angle of light hitting those iridophores, the same structural-color trick behind neon tetra blue just tuned to a different wavelength. Mature males grow a trailing extension off the center of the caudal fin that females never develop, an easy field mark once you know to look for it. Deep-bodied and unhurried compared to smaller tetras, it reads as a calm anchor in a mixed school.

Care level: Easy · Temperament: Peaceful, shoaling · Max size: ~2" · Min tank: 20 gal · Diet: Omnivore, flake/micro-pellet