The leopard danio is the spotted pattern form of the zebra danio, and it shares that fish's defining trait: it almost never stops moving. Adults reach about two inches and swim fast, high in the water, in a loose, quarrelsome, joyful school. Their minimum tank is a 20-gallon long - and the 'long' is doing the work in that sentence. A tall 20-gallon with a small footprint short-changes them badly, while a long tank of the same volume gives the school the straight-line runs it spends all day making. They are also among the hardiest schoolers you can buy, which makes the tank-length mistake the main thing to get right.
Length is the real requirement
Danio keeping goes wrong when gallons are counted instead of inches. A leopard danio school uses a tank the way a greyhound uses a field - in sprints, chases, and sudden turns - and it needs a clear run of roughly three feet to do it naturally. Height, by contrast, is nearly wasted on them - they live in the top half of the water. Set the tank up with that run kept open along the front and middle, plants and wood massed at the ends, and the school will spend the day racing the length of it. Fit a tight lid without gaps: danios launch themselves at feeding time and during chases, and a fish this fast finds the one open corner.
Cool water and a full school
Leopard danios are subtropical fish, comfortable from the mid-60s to the mid-70s Fahrenheit, which makes them one of the few schooling fish genuinely happy in an unheated room-temperature tank. Warmer tropical water speeds them up and shortens their lives, so do not file them in a heated community by default. Keep at least six, and expect constant low-level squabbling - that is normal danio society, spread harmlessly across a group. A group of eight to ten in a long tank is a reliably better sight than six. In pairs the same behavior concentrates on one fish, which is why danio groups should never be bought as twos and threes.
Tank mates that can keep up
Match leopard danios with fish that share their pace and their temperature: white cloud mountain minnows, other danios, and fast, peaceful barbs all work, as do corydoras along the bottom. Slow, long-finned fish are the wrong company - they get outcompeted at every feeding and may have their fins investigated by curious danios. Bottom dwellers that need strong current are also a mismatch, since the danios are happiest in gentle flow. In a cool-water community, check every candidate for the temperature overlap first and the temperament second; with danios, most failures are thermometer failures.
Checking the temperature overlap before buying
The usual way a leopard danio tank goes wrong is a well-meant addition that secretly needs tropical warmth, bought because it looked peaceful in the shop. Check the overlap before the bag is tied, not after. If a species' temperature range is unclear, put a follow-up question to KinsLe's Kimmy; she answers using the KinsLe Answers library.