Stenolicmus ix can be distinguished from S. sarmientoi by the length of the nasal barbels, reaching the base of the first opercular odontodes (vs. reaching anterior margin of eyes); length of maxillary barbels reaching posterior margin of opercular odontode plate (vs. reaching posterior margin of interopercular odontode plate); seven well-developed opercular odontodes (vs. six); seven well-developed interopercular odontodes (vs. five or six); pelvic fin rays, i+4 (vs. iv); color pattern of posterior region of trunk composed of agglomerated chromatophores forming circular patches twice the diameter of the eye (vs. without circular patches twice the diameter of the eye); large eyes with 11.8% HL (vs. 6%); caudal peduncle depth, 11.6% SL (vs. 8.5–9.5%); absence of dark bar at the base of the caudal fin (vs. presence of dark bar); head length, 17.9% SL (vs. 14.9–15.1%); and unbranched rays of caudal fin long, reaching distal margin of fin (vs. rays short, not reaching half the length of caudal fin) (Ref. 86403).