Diagnosis: Nothobranchius skeltoni is distinguished from all other species of the genus by the following characters in males: body colouration light blue with grey to brown scale margins; anal fin grey to red-grey with irregular dark grey spots proximally, followed by a broad dark red to maroon medial band, a narrow light grey subdistal band, and a broad black distal band; caudal fin light grey with slender semi-translucent dark grey distal band; pelvic fins light blue-grey, with broad dark red to maroon medial band and narrow black distal margin (Ref. 122068). Furthermore, it differs from the other members of the Nothobranchius taeniopygus species group in central and western Tanzania by a body depth at pelvic-fin origin 26.3-30.0% of standard length vs. 30.3-37.6%; suborbital depth 15-22% of head length vs. 23-31%; caudal peduncle depth 11.8-12.9% of standard length vs. 12.9-16.8%; and caudal peduncle length 160-169% in its depth vs. 119-151% (Ref. 122068).

Description: General body shape robust, laterally compressed and moderately deep; greatest body depth at vertical in front of pelvic-fin origin, 26.3-30.0% of standard length; greatest body width at pectoral-fin base with body progressively narrowing towards caudal-fin base (Ref. 122068). Dorsal profile convex from tip of snout to base of last dorsal-fin ray, straight to slightly concave on caudal peduncle; ventral profile convex from lower jaw to base of last anal-fin ray, straight to slightly concave on caudal peduncle; caudal peduncle relatively shallow, length 1.6-1.7 times its depth; anus situated directly in front of anal-fin origin (Ref. 122068). Head modestly long, 27.4-33.7% of standard length, laterally compressed, deeper than wide, head width 66-73% of its depth; snout slightly pointed, smaller than eye diameter; mouth supraterminal, slightly oblique in profile; jaws subequal, lower jaw longer than upper, posterior end of rictus at same level or slightly ventral to centre of eye; premaxilla and dentary with many irregularly distributed conical, slightly curved teeth at outerrow of lower and upper jaws; orbit large, 28-35% of head length, mostly in anterior half of head, snout to eye end length 46-54% of head length, in dorsal portion of head side; branchiostegal membrane projecting posteriorly from opercle (Ref. 122068). Dorsal-fin origin anterior to anal-fin origin, both fins originating posterior to mid-length of body; extremity of dorsal and anal fins rounded, with small contact organs in form of papillae on fin rays and distal margin with short filamentous rays; posterior extremity of dorsal fin reaching caudal-fin base; dorsal-fin 15-17 rays; anal fin 15-17 rays; pectoral fin subtriangular, insertion slightly posterior to margin of opercular opening, base slightly oblique, upper fin rays placed slightly anteriorly to lower fin rays, tip reaching or slightly overlapping base of pelvic fin; pelvic fin subabdominal, origin at about mid-length of body, short, bases medially separated, tip reaching urogenital papilla; caudal fin subtruncate, with 16-18 branched rays, plus 2 or 3 unbranched smaller rays at dorsal and ventral origins (Ref. 122068). Scales cycloid, body and head entirely scaled, except for ventral surface of head; no scales on dorsal and anal fins; scales in mid-longitudinal series 28-31 plus two or three small scales on caudal-fin base; transverse rows of scales in front of dorsal-fin origin 11-12; scale rows around caudal peduncle 12-14 (Ref. 122068). Cephalic squamation pattern variable; frontal neuromasts separate in two rows of shallow grooves behind the upper lip, with one neuromast in each groove; cephalic sensory system at supraorbital level fragmented into two discontinuous portions of shallow grooves, with two and three exposed neuromasts in anterior and posterior portions, respectively; supratemporal level with two exposed neuromasts and five small buttons in undivided, slightly curved shallow groove; preorbital canal fragmented into three portions of shallow grooves with two exposed neuromasts in each portions; infraorbital level with series of about 15 small buttons at ventral and posterior margin of orbit; postorbital canal in deep groove with three exposed neuromasts; preopercular system with exposed neuromasts in deep groove ; mandibular canal in shallow groove with about five small neuromasts; one neuromast on each scale along trunk mid-longitudinal series (Ref. 122068).

Colouration: Live male: scales on trunk and head light iridescent blue to blue-grey with broad dark brown to dark grey margins, forming a regular reticulation pattern; scales on abdomen faint blue to white with less pronounced margins; scales on the dorsum anterior to the dorsal fin are grey to brown; snout, frontal and dorsal portions of head blue-grey; throat pale blue-grey; exposed part of branchiostegal membrane very pale brown; iris golden; dorsal fin blue-grey to grey-brown with irregular brown to red-brown dots and stripes, aligned parallel to the rays, becoming finer distally; small black markings may be present on membrane between first three anterior fin rays; anal fin with grey to red-grey basal and proximal zones overlaid by brown dots; medial zone broad dark red to maroon band followed by very narrow, poorly developed light grey subdistal band and broad black distal band; bulk of caudal fin light grey with slender, dark grey semi-translucent distal zone; specimens from type location show a very poorly-developed, narrow and very faint, light grey subdistal zone; pelvic fins mimic pattern of anal fin with narrow grey basal zone, broad red to red-brown medial zone and black distal band; pectoral fins dominantly hyaline with light blue margins (Ref. 122068). Live female: trunk and head light grey-brown, darker on dorsum and lighter ventrally; scales with a very narrow grey margin; scale centres iridescent silver to very pale blue; all fins hyaline; iris golden (Ref. 122068).