Small fish, long tank
Zebra danios top out around two inches, yet their minimum tank is set by speed rather than size: they are relentless, all-day swimmers that run the length of the tank in packs, chase, display and double back without pausing. A short tank turns that energy into frantic wall-pacing. Ten gallons is the bare floor for a small school, and only in its long form; a twenty-gallon long is the size that actually suits them, giving the school the straight runs their behaviour demands. In the wild they live in clear, flowing streams, and everything about their build — slim, torpedo-shaped, restlessly facing into movement — reflects that origin. Height is nearly wasted on danios — they live in the top third of the water, feed at the surface and use every inch of length you give them while ignoring the depths below.
Numbers, and the school they form
Six is the minimum school, and eight to ten is better: below that, danios grow nippy and insecure, picking at each other and at any slow tankmate within reach. A proper school burns its energy internally, chasing and displaying among themselves, and becomes a single shimmering organism rather than a set of restless individuals. Males in breeding condition deepen in colour and pursue females persistently at first light, which is another reason the school should be generously sized — in a large group that attention is shared around until it dissolves, while in a small one it fixes on a single fish until she is exhausted. They are also famously hardy and tolerate cooler, unheated-room temperatures better than most tropical fish, which is much of their beginner reputation. But hardy is a cushion for mistakes, not a reason to make them — and the swimming-space requirement is the one thing hardiness does not soften, because a cramped danio is a stressed danio however clean the water is.
Lids, flow and the top of the tank
Danios jump: a startled school leaves the water together, and a tank without a close-fitting lid will lose fish through gaps you would swear were too small. Many keepers give them a gentle current to swim against, which they visibly enjoy, streaming into the flow and holding station like tiny trout — a filter outflow aimed along the tank's length turns the whole top layer into their racetrack. Keep long, low planting along the back and edges so the open front stays clear for swimming, and avoid pairing them in a minimum tank with slow, long-finned fish whose trailing fins invite investigation by a school with energy to spare.
One stocking list for the whole house
A danio school changes by small degrees — a fish lost here, three added there — and those changes are exactly what gets misremembered when someone buys on impulse at the shop, producing duplicate top-ups nobody planned. Keeping the current school count, tank size and feeding notes on the profile the whole household sees through KinsLe Family Sharing means every addition is a decision made against the real numbers, not a guess made at the till.