Moenkhausia parecis can be diagnosed from all congeners, except M. clorophthalma, some populations of M. cotinho, M. lineomaculata, M. petymbuaba, and M. plumbea, by having a dark blotch on the anterior portion of each scale of the second to seventh longitudinal series (vs. pigmentation absent or, when present, concentrated at the posterior margin of scales, forming a reticulate pattern). It is easily distinguished from all aforementioned species by having completely blue eyes in life (vs. green in M. clorophthalma, mostly green with some red in M. petymbuaba, lower portion blue and upper portion orange in M. lineomaculata, clear or red in M. cotinho, and clear, with a longitudinal dark stripe in M. plumbea). It differs also from M. clorophthalma, M. petymbuaba and M. plumbea by having 15-18 (rarely 18) branched anal-fin rays (vs. 18-24), from M. cotinho and M. lineomaculata by having a smaller caudal-peduncle spot, with only the base of the middle caudal-fin rays pigmented (vs. blotch larger, base of all caudal-fin rays pigmented in M. cotinho and M. lineomaculata, except the outermost unbranched rays in some specimens of M. lineomaculata) and by the absence of a light area preceding caudal peduncle spot (vs. presence of a light area preceding caudal-peduncle spot). It further differs from M. clorophthalma, M. petymbuaba and M. plumbea by the presence of a well-defined, round caudal-peduncle spot, that does not extend to the tip of the middle caudal-fin rays (vs. caudal-peduncle spot absent or poorly defined in M. clorophthalma and M. plumbea or caudal-peduncle spot confluent with longitudinal stripe on body, reaching the tip of middle caudal-fin rays in M. petymbuaba) (Ref. 109906).