Betta falx Tan & Kottelat 1998

This species is closely related to Betta picta from Java where it can be distinguished in the following characters: fewer dorsal fin rays (mode 8 vs. 9); fewer lateral scales (mode 27 vs. 28); dorsal fin origin above 11-12th lateral scale (vs. 12-14th); anal fin origin below modal 6th lateral scale (vs. 7th); fewer predorsal scales (mode 19 vs. 20); slightly greater anal-fin base length (46.5-50.3% SL vs. 42.6-48.4); anal and caudal fins with reddish distal band in live male (vs. bluish); iridescent greenish-blue opercle scales (vs. yellow-gold); in preserved specimens, male with distinct dorsal transverse bars (vs. faint); male with distinct dark anal distal margin wide (vs. narrow); male without elongated median caudal fin rays (vs. presence); female with distinct caudal transverse bars (vs. very faint or absent); dorsal head view narrow (vs. broad); thick preorbital black stripe (vs. narrow); and the distance between posterior part of anal fin to lower part of caudal narrow (vs. wide) (Ref. 56386).

Dorsal spines (total): 1 - 2; Dorsal soft rays (total): 7 - 8; Analspines: 2; Analsoft rays: 19 - 21

Typically found among submerged bank vegetation, in near stagnant waters, with pH 4.7-6.8. Latin word falx means scythe, alluding to the continuous curved shape of the broad anal and caudal distal margins of a male in display. Caudal fin rounded.

Betta falx is a species of betta endemic to the island of Sumatra in Indonesia. It is an inhabitant of acidic, nearly stagnant water, where it can be found amongst the vegetation along the banks. This species grows to a length of 3.5 cm (1.4 in).