It also doesn't help that both are really variable in coloration depending on conditions / the environment.
In the end, I think that they're really, really closely related species without a distinctive anatomical feature that is unique. So natural range and collection location becomes the distinguishing characteristic...but that's not necessarily known for tank-raised fish.
Some discussion from
www.cichlidae.com:
Diagnosis: C. myrnae differs from
Cryptoheros septemfasciatus in (1) The presence in both sexes of parallel rows of metallic yellow dots on the scales of the lower half of the flanks, (2) the absence of complete dark bars on the flanks in both living and preserve specimens, (3) the presence of small iridescent blue spots in the soft dorsal and median zone of the caudal and (4) the presence in the female of a trapezoidal dusky zone extending from the origin of the ventral fins upward to a point one scale row above the pectoral fin origin and rearward to the posterior lower margin of the midlateral spot and downwards to a point just above the anal fin origin. It differs from
C. nanoluteus in a more rounded dental arcade in both jaws, the absence of dark lateral bars and the suppression of metallic yellow coloration in parental individuals of both sexes (Compiled from Loiselle, 1997).
No unique autapomorphies, but distinguished from other species of the subgenus
Bussingius as follows: upper symphysial teeth usually abruptly larger than adjacent teeth (vs. not abruptly larger); gill rakers on first arch digitiform, blunt (vs. trapezoidal or bifid); a diffuse but complete longitudinal stripe, ending in a tenuous blotch on the caudal peduncle (vs. on the fin); lateral spot circular (vs. oval); predorsal scales modally 11 (vs. modally 12 or more) (Compiled from Schmitter-Soto, 2007).
And the ultimate source of the confusion:
Comments: It is not clear when
Cryptoheros septemfasciatus was introduced into the aquarium hobby, Stawikowski & Werner (1985: 257-258) show aquarium pictures of both
C. septemfasciatus and
C. myrnae. At that time, the recently discovered
Cryptoheros myrnae from the Sixaola river was considered a color morph of
C. septemfasciatus. Stawikowski & Werner also mention that in the United States, both an albino and a golden morph were available, which indicates that the fish had been around at least for some years already. I however have not been able to find older records of their keeping.
Matt