Diagnosis: Aphyosemion grelli is distinguished from all congeners by possessing a unique colour pattern of the unpaired and pelvic fins in females consisting the combination of a yellow basal portion and greyish to dark grey broad margin vs. never a similar colour pattern (Ref. 94461). Males are similar to Aphyosemion congicum, A. labarrei, A. ocellatum, A. passaroi, and A. teugelsi and distinguished from all other species of the genus by having black margins on their unpaired fins vs. never a similar colour pattern (Ref. 94461). Aphyosemion grelli differs from A. congicum males by the blue-green body and head colouration vs. orange; by the rounded dorsal and anal fins vs. pointed; by a horizontal series of densily set dots on flank vs. a few, larger and sparsely set dots; by a thin black distal margin on dorsal fin vs. dorsal fin almost completely black; by the rounded caudal fin vs. caudal fin pointed dorsally and ventrally; and by 11-12 dorsal-fin rays vs. 9-10 (Ref. 94461). Aphyosemion grelli differs from males of A. labarrei by a horizontal series of dots on flank vs. a dense pattern of large red blotches forming fused, irregular bands on caudal fin; by the rounded caudal fin vs. pointed dorsally and ventrally; by 10-12 dorsal-fin rays vs. 12-14; by 13-14 anal-fin rays vs. 14-17; and by 12 circumpeduncular scales vs. 13-14 (Ref. 94461). Aphyosemion grelli differs from males of A. ocellatum by a horizontal series of dots from humeral to caudal-fin base vs. single blue broad rounded blotch at humeral region; by the more anterior insertion of dorsal fin relative to anal-fin origin (D/A 6-7 vs. D/A 8-9); and by 13-14 anal-fin rays vs. 15-16 (Ref. 94461). Aphyosemion grelli differs from males of A. passaroi by the presence of a horizontal series of dots on flank vs. absence or a more irregular pattern of a few smaller dots on flank; by caudal fin with narrow black dorsal and ventral distal margins vs. broad black band along entire margin of caudal fin; by anal fin without any spots vs. with a pattern of red dots; and by 13-14 anal-fin rays vs. 15-16 (Ref. 94461). Aphyosemion grelli differs from males of A. teugelsi by the light pectoral-fin margin vs. dark; rounded dorsal and anal fins vs. pointed; by 16-18 pectoral-fin rays vs. 14-15; by 28-32 caudal-fin rays vs. 23-25; and by 9-10 scales on transverse series vs. 7-8 (Ref. 94461).

Description: Body slightly laterally compressed; dorsal profile slightly convex, maximum body depth at pelvic-fin origin; ventral profile slightly convex, slightly concave to nearly straight on caudal peduncle (Ref. 94461). Snout slightly rounded, mouth directed upwards, lower jaw longer than upper jaw (Ref. 94461). Caudal fin subtruncate; dorsal and anal fins located posterior to mid-body, tips rounded (Ref. 94461). Frontal neuromast series 'open' type; supraorbital series with three neuromasts; preopercular canal with six pores; one neuromast on each scale of median longitudinal series (Ref. 94461). Scales cycloid, body entirely scaled except ventral surface of head; frontal squamation G-type; scales on mid-longitudinal deries 28-31; scales on transverse series 9-10; scales around caudal peduncle 12 (Ref. 94461). Dorsal-fin rays 10-12, first dorsal-fin ray on vertical between 6th to 7th anal-fin ray; anal-fin rays 13-14; caudal-fin rays 28-32; pectoral-fin rays 16-18; pelvic-fin rays 5-6 (Ref. 94461). Premaxilla and dentary with unicuspid, slightly curved teeth; vomerine teeth absent, anterior border of vomer thickened; post-temporal lateral process absent; anterodorsal process of the urohyal bifid (Ref. 94461).

Colouration: Male. Body and head metallic blue-green, three interrupted horizontal series of red dots on flank; two narrow oblique red bars on opercle (Ref. 94461). Dorsal, anal and pelvic fins blue-green with narrow black distal margin; caudal fin blue-green with dark blue irregular dots, dorsal and ventral distal margin black, dorsal and ventral posterior portion white; pectoral fin hyaline with ventral white margins (Ref. 94461).

Female. Body and head grey-brown, three interrupted horizontal series of small red dots on flank (Ref. 94461). Pectoral fin hyaline; dorsal, anal, caudal and pelvic fins yellow with broad grey distal margin (Ref. 94461).