Two to two and a half inches of spots
An adult leopard danio finishes at about two to two and a half inches long, with females growing a little deeper through the belly than males. The leopard is a pattern form of the zebra danio, so instead of horizontal stripes the flanks carry a scattering of dark spots over a gold-silver body, and no two fish wear exactly the same arrangement. Long-finned forms are common in shops and look considerably bigger, but that is all fin: the body underneath still stops growing in the same modest range, which is worth remembering when judging how much space a group really occupies.
The most active two inches in the tank
Adult size understates what leopard danios demand, because few fish this small use as much water. They race along the surface and upper midwater in near-constant motion, chasing, displaying, and wheeling as a group, so a long tank gives them the straight runs their behaviour is built around while a short tank turns the energy into restless circling. A tight lid is non-negotiable, since a startled or displaying danio leaves the water easily. Kept in a proper group, their chasing stays within the school and reads as play; kept nearly alone, a danio will direct the same restlessness at slower tankmates.
Cool, flowing water suits a danio's build
Leopard danios come from cooler, moving waters, and their adult care is easiest when that background is respected. They stay lean, bright, and tireless at the cooler end of the tropical range, in a gentle current that gives the school something to swim against, and they tolerate the mild temperature swings of an unheated setup better than most fish their size. Warm, still water makes the same fish restless in the wrong way, shortening their active periods and dulling the spotting. Position filter flow along the tank's length rather than across it, and the school will spend the day surfing its own private river.
Feeding fish that burn everything they eat
Leopard danios feed at the surface with quick, snapping bites and a metabolism to match their movement. Small floating foods suit them best, offered where the school can sweep through as a group, and they benefit from the food being spread along the surface rather than dropped in one spot the boldest fish can guard. Because they are so active, condition shows as a clean, streamlined body rather than bulk; a danio that becomes sluggish and sits out the group's racing is visibly out of step with its species and worth watching closely at feeding and rest.
Tankmates that can live at danio pace
Peaceful but relentless, leopard danios suit companions that are untroubled by constant motion overhead: other active schoolers, robust midwater fish, and bottom dwellers that ignore the traffic entirely. Slow, shy, or long-finned fish make poor partners, stressed by the commotion and outpaced at the surface at every feeding. Hardy across a wide range of conditions, danios still colour and behave best in a settled, roomy setup rather than the bare minimal tank their size seems to permit. Note the group's size and your tank's length in the profile you keep for them, since those two facts explain their behaviour better than anything else. If the group is passed on, KinsLe's Transferrable Profiles let the leopard danios' profile transfer with their history and routines intact.