Diagnosis: Nothobranchius niassa belongs to the subgenus Adiniops, which has a preopercular red pattern, wide chevrons on side and a deep red caudal fin without any dark margin in males (Ref. 91043). It is distinguished from all other species of this subgenus by the unique combination of the following characters in males: dorsal fin rays 15-18, anal fin rays 15-17, light blue iris, light blue dorsal and anal fins with curved red stripes, and red pectoral fin (Ref. 91043).

Description: Somewhat elongate Nothobranchius species; dorsal profile nearly straight on head, convex from nape to end of dorsal-fin base; ventral profile convex, slightly concave to nearly straight on caudal peduncle posterior to dorsal and anal fin (Ref. 91043). Snout slightly pointed, mouth directed upwards, lower jaw longer than upper, posterior end of rictus at same level as or slightly above centre of eye (Ref. 91043). Branchiostegal membrane projecting posteriorly from opercle (Ref. 91043). Dorsal and anal fins posterior to mid-body, rounded, fin tips with short filamentous rays; tip of dorsal fin reaching caudal fin; dorsal and anal fins covered with an opaque mucus film and with papillate contact organs along fin rays, denser on anal fin; pectoral fin approximately triangular, tip reaching pelvic fin; pelvic fin long, tip reaching anal-fin origin; caudal fin subtruncate; dorsal-fin rays 15-18; anal-fin rays 15-17; caudal-fin rays 26; pectoral-fin rays 16; pelvic-fin rays 6 (Ref. 91043). Scales cycloid, body and head entirely scaled, except for ventral surface of head; scales in median lateral series 27-30+3-4 on caudal-fin base (Ref. 91043); cephalic squamation pattern irregular G-type (Ref. 91043). Anterior neuromast series of 'open' type; central supraorbital series in shallow groove with 2-3 neuromasts; posterior cephalic neuromast series curved with 2-3, rarely 4, neuromasts; preopercular neuromast system in open groove, distal ridge slightly overlaps opercle; one neuromast on each scale of median longitudinal series (Ref. 91043).

Colouration: Males with body and head scales light blue with a dark red margin, creating a reticulated pattern on body and head, posterior scales margin width on anterior part of body; posterior part of caudal peduncle, lips, snout, frontal, and dorsal portion of head red; branchiostegal membrane red with light blue rim; dorsal and anal fins red with a pattern of light blue spots, forming irregular stripes; pelvic fin red with a pattern of rounded light blue spots; pectoral fin red with light blue margin; caudal fin red; iris light blue, with faint black vertical bar through centre of eye (Ref. 91043). Females with body and head scales light brown, scales on sides with light blue to silvery centers, ventrally whitish; opercular region silvery to whitish; paired and unpaired fins hyaline; iris silvery, with faint black vertical bar through centre of eye (Ref. 91043).