Hyphessobrycon serpae, one of the Serpae tetras

Hyphessobrycon serpae or eques (Serpae Tetra).  Only minor differences in shoulder blotches and coloring distinguish Hyphessobrycon serpae from H. bentosi, H. callistus, H. copelandi, H. minor and other populations, sub-species or species. In a 1997 Tropical Fish Hobbyist article Stanley Weitzman relegated the familar specific names callistus and serpae to the status of  junior synonyms of Hyphessobrycon eques described by Franz Steindachner in 1882. Other hobby-oriented articles and some professional ones followed suit. Such an abundance of synonyms is not simply confusing: it represents the fact that the Serpae Tetras form a species flock, formed of populations with subtly distinctive coloring. Marks and blotches appear more strongly in young, healthy specimens , which may be identified by some researchers as sub-species. The various wild populations of these "Blood Characins", as German aquarists first called them, have been blurred by generations of interbreeding in captivity since these fishes first appeared in German and US aquaria in the 1920s; then they were identified both as Hyphessobrycon serpae and H. callistus. As long ago as the 1950s Gunther Sterba noted that in aquarium-bred  fish, the same spawning would produce individuals with distinct shoulder-blotches together with ones in which the blotches were faint or absent, and much variation in the black of the dorsal fin. 
 
These active tetras may harass slower-moving fish, even motionless Otocinclus. They should be kept in groups of six or more.  
 
An account of breeding the Serpa Tetra is Brian Cole and Michael Haring's  "Spawning and production of the Serpae Tetra, Hyphessobrycon serpae", 1999.
 
Serpae Tetra at Wikipedia.