Zebra danio sexing is a side-by-side judgement
No single zebra danio carries an obvious male or female badge, so the method is comparison within the shoal. Spread your eye across the group at feeding time and sort them by silhouette: the rounder, deeper-bellied fish are females, and the slim, straight-bodied ones are males. A lone danio, or youngsters of mixed ages, defeat the method because there is no honest baseline. This is one more reason danios belong in a proper shoal of their own species, where the contrast between the sexes stands in the same water, eating the same food, under the same light.
The golden tint between the stripes
Colour supplies the danio's second clue, in the pale bands running between the dark stripes. In males those bands often carry a warm golden or yellowish tint, giving the whole fish a slightly gilded look when it turns in the light. In females the same bands read cooler and more silvery-white. The tint strengthens in males that are settled and in good condition, and stress or shop lighting can wash it out entirely, so judge colour on fish that have been home and feeding well for weeks. On a young danio the tint is embryonic at best, another reason patience outranks squinting.
The female's belly, especially with eggs
A mature female zebra danio is built around her belly. Even unbred, she looks fuller and rounder than any male, and when she is carrying eggs the swelling becomes unmistakable, her underside bulging gently and looking paler. Males never show this profile at any feeding level, staying narrow from gill to tail. Judge bellies in the morning before the day's food rounds everyone out, and compare fish of similar length, because a big old male can outweigh a small young female while still being the slimmer shape. Shape, not size alone, is the danio rule.
Chasing at dawn is a supporting scene
Zebra danios are dawn spawners, and their early-morning behaviour sketches the sexes in motion. Males chase females persistently through plants and across open water in the first light, the slim golden fish doing the pursuing in tight, excited circuits. Females scatter eggs among fine leaves when breeding actually happens, though in a community tank the eggs are eaten almost immediately. Young danios chase in play as well, so an afternoon squabble sexes nobody; sustained dawn pursuit by slim fish of rounder ones, repeated over days, is the pattern worth noting in your records.
Why a keeper bothers with danio sexes
A balanced danio shoal behaves better than an accidental crowd of one sex, since groups heavy with males can turn dawn chasing into all-day harassment when females are few. Anyone hoping to raise danio fry absolutely needs the comparison skill, because a breeding setup starts with selecting a plump female and a slim, gold-tinted male. Let your youngsters grow to full size before sorting them into either story. Recording the outcome fish by fish in a KinsLe Pet Passport means each danio's sex, batch, and breeding history sit in one portable place when the shoal is moved or passed on.