Pigment helps separate S. compressus from S. altuvei. Young S. compressus have larger and denser spots that extend more fully over the ventral body and belly than in S. altuvei, where spotting is sparse below the lateral line. (Fink, et al. 2001). The body color is gray metallic, the anterior thru basal of the anal fin is reddish with a black border. The caudal fin has a terminal black band. The other fins are hyaline. Some individuals present a black humeral spot. (Jégu, M., E. L. M. Leão and G. M. dos Santos, 1991). Eyes are reddish-orange. Ectopterygoid teeth common to Serrasalmus species are visible in this photo (upper left) of the upper palate of S. compressus.
If that's true then my fish looks more like an altuvei, but my dealer said it was shipped from Brazil, so most likely its a compressus. Look at these two pics though: