This tetra is one of some 30 species commonly referred to as the "rosy tetra clade" within the genus Hyphessobrycon that share several traits in colour and pattern. All species are somewhat disk-shaped and share the "flag" signal, being a very conspicuous black spot on the dorsal fin, usually underlined by a white or sometimes yellow zone and tipped with white depending upon species. They also share a darkened humeral or shoulder patch immediately posterior of the gill covers. Several have a red or mauve body tone (some like the Black Phantom do not).
From your photo I would suggest this species may be Hyphessobrycon rosaceus, the Rosy Tetra. The ornate tetra, H. bentosi, is very similar. And fish under stress (as they would be if newly acquired or if the water/environment is not suited) are often paler or less distinct in identifying traits. There are new species sharing many of the same traits appearing in the hobby making ID even more difficult.
What did the store say this species is?
EDIT. As I was typing, Naughts posted; Serpae Tetra (Hyphessobrycon eques) is certainly one of the species in this "Rosy clade" but not the one pictured in my view.