When I bought my school of Bleeding Heart Tetra, this little guy ended up with them. I have no idea what it is, despite spending hours scouring over images of Tetra, Pencilfish, etc. The closest I've come to identifying it is a Blue Tetra, but they lack the red gill plates.
It's quite small, but larger than a Cardinal or Neon. Very fast swimmer, and not timid/shy at all. The pectoral fins, which are transparent also point upward, rather than outward like the Tetra. At night, the body goes almost transparent, save for a tiny light blue lateral line mid-body, and red gill plates. During the day, it goes a darker blue-grey. The top of the eyes are turquoise (unlike the Blue Tetra, which are yellow).
It also seems to enjoy nibbling on the blanched spinach I put in for the Otos, but leaves the actual live plants alone.
Any ideas what it is? I'd like to get it some schooling buddies.