Hyphessobrycon diastatos is distinguished from all congeners, with the exception of H. amandae, H. axelrodi, H. brumado, H. compressus, H. diancistrus, H. eilyos,H. georgettae, H. heteresthes, H. gracilior,H. milleri, H. minimus, H. negodagua, H. otrynus, H. parvellus, H. piabinhas, H. procerus, H. saizi,H. scutulatus, H. taurocephalus, and H. tukunai by lacking any concentration of chromatophores at the humeral region (vs. presence of a dark humeral spot or/and a longitudinal dark stripe extending over the humeral region). It differs from H. otrynus, H. piabinhas, and H. procerus by having 15-18 branched rays on anal fin (vs. 19 or more); from H. amandae, H. axelrodi, H. compressus, H. georgettae, H. heteresthes, and H. milleri by the absence of a dark blotch in the dorsal fin (vs. presence); from H. eilyos and H. scutulatus by having 1-3 maxillary teeth (vs. 5 or more); from H. diancistrus by having hyaline caudal-fin lobes (vs. caudal lobes with a black spot); from H. gracilior by having 4-5 longitudinal scale rows below the lateral line (vs. 3); from H. minimus by having the largest dentary teeth with 3-5 cusps (vs. 7); from H. saizi by having 14 horizontal scale rows around caudal peduncle (vs. 12) and 10-12 predorsal scales (vs. 9); from H. taurocephalus by having adipose fin (rarely absent, 1 of 294 specimens) and 15-18 branched rays on anal fin (vs. adipose fin absent and 12-15); and from H. tukunai by having 2-5 teeth in the outer premaxillary teeth row (vs. 1, rarely 2), besides the maxillary teeth with 1-3 cusps (vs. 5 or more). It is distinct from H. brumado, H. negodagua, and H. parvellus by the round and relatively well-defined dark spot on caudal peduncle in males (vs. caudal peduncle blotch with inconspicuous border in males, frequently only represented by dense pigmentation on this region) and also by the absence of white pigmentation on distal portions of first rays of dorsal, anal, and caudal fins in life (vs. presence). It further differs from H. negodagua and H. parvellus by having dark pigmentation of caudal-peduncle spot extending to base of few middle caudal-fin rays or reaching their tip, never on caudal lobes in males (vs. caudal-peduncle pigmentation spread from middle caudal-fin rays to inner portions of caudal-fin lobes in males); from H. brumado and H. parvellus by having pectoral, pelvic, and anal fins hyaline to slightly yellowish in life (vs. distinct orange- to reddish) and sexually dimorphic elongation of dorsal and anal fins in males (vs. absence). It can be further separated from H. brumado by having overall silvery body color in life (vs. orange- to reddish) and from H. negodagua by the absence of dense concentration of dark chromatophores over dorsum and posterior portion of body (vs. presence) and by the presence of adipose fin (rarely absent, 1 of 294 specimens) (vs. usually absent; and 4 of 158 with adipose fin (Ref. 102657).

Description: Dorsal-fin rays ii-iii,8-9; anal-fin rays iii-iv,15-18; pectoral-fin rays i,9-11; pelvic-fin rays i,6; longitudinal scale rows between lateral line and pelvic-fin origin 4 (Ref. 102657).