Hmmm.... id'ing juvenile serras from a picture alone, will always be more guess than fact.
It could be a rhomb. But the terminal band on the caudal-fin seems somewhat transparent, this could point towards S.sanchezi.
I also see some oval appearing spots on this specimen, this along with the coloration of the gill-covers and analfin, could tell it's a S. gibbus if it's from the Rio Araguaia, and if the terminal band of the caudal-fin is only obscured by the quality of the picture.
It all depends on origin, and present size of the fish.