Ten to twelve as the full school
A 20-gallon tank is where zebra danios stop being a compromise. Ten to twelve fish form a school with real depth — enough individuals that the group flows, splits and regroups along the glass instead of stringing out in single file — and the tank still leaves them unbroken swimming length. If the danios are the tank's only fish, twelve is a good number to settle on. If they will share the water, eight danios is the better school, with the remaining capacity spent on fish that live at levels the danios largely ignore.
Buy the length: long beats tall
Twenty gallons comes in two very different shapes. A 20-gallon long gives danios a racing lane roughly three feet end to end; a 20-gallon tall gives them headroom they will never use, because danios live in the top and middle of the water column. Choose the long footprint, keep a tight lid on it — a dozen startled danios hitting the surface will find any gap you leave — and landscape the back with plants while the front stays an open lane. A gentle current along the tank's length suits these stream fish, and the school will swim into it happily for hours.
Tankmates that use different water
The danios own the upper and middle levels, so companions should live underneath them. A group of corydoras catfish working the bottom, a small pleco, or a colony of cherry shrimp in the planted corners all fit without competing for the lane. Avoid other streamlined schoolers in the mid-water — the tank becomes two queues for the same space — and avoid slow, long-finned fish whose food the danios will steal and whose fins they may nip when the school is small or bored. Stocking the danios at full school size is itself the best behaviour insurance you can buy.
Feeding a fast school
Danios eat at the surface and eat quickly, so scatter small foods along the front glass rather than dropping one clump that the boldest three fish monopolise. One or two light feedings a day, sized so the school finishes within a minute or two, keeps the water cleaner than a single heavy meal. In a shared tank, drop sinking food for the bottom fish after the danios have had their first rush, or the corydoras will spend their lives living on crumbs.
Planning the community as a whole
Community tanks usually fail by accumulation — one reasonable addition at a time — rather than by any single bad choice. Before adding the next species, put a follow-up question to KinsLe's Kinny about the whole community plan; he answers using the KinsLe Answers library.