This is one of two species commonly imported under the moniker "Rocket Pencilfish," due to their habitat of facing the surface, as though they are ready to blast off!
@Osedax is right, it's the one with the checkered pattern. That species is the Hockeystick Pencilfish (Nannostomus eques), which is sometimes found schooling with N. unifasciatus, and vice versa. These are two of I believe 3 or 4 species often sold under the moniker "Rocket Pencilfish."
