Small fish with a social structure
The celestial pearl danio - the galaxy rasbora, to many shops - is a fish barely an inch long, patterned with pale spots on a dark body and, in the males, fins edged in vivid orange-red. It comes from shallow, plant-choked ponds, and its social life runs on display: males circle each other with fins flared, posture and spar for status, and court females in the vegetation. None of that behaviour exists in a pair. A small group of these fish is not a diluted version of a large one; below a certain number the whole social system switches off, and what you keep instead is a few anxious fish hiding in the plants.
Numbers, and the ratio inside the numbers
Six is the floor for a group, and eight to ten is the range where celestial pearl danios relax and start performing. Within that number, the ratio matters as much as the total: keep more females than males. Males direct their sparring at each other and their courtship at the females, and with too few females, one or two get pursued without rest while the males wear themselves out competing. A group of, say, three males and six females spreads every kind of attention comfortably. This is one of the species where buying 'six assorted' can quietly go wrong - it is worth asking the shop to pick a female-heavy mix, or buying a slightly larger group so the odds settle kindly.
Space and planting for a shy fish
The good news is that a proper group does not need a big tank: a well-planted nano aquarium suits a single group of eight to ten, since their combined size is tiny. What they need from the space is structure - dense, fine-leaved plants and moss to retreat into, with a small open patch where the males can display. Cover is what makes them brave: a celestial pearl danio that knows it can vanish into the plants in one flick will spend its day out in the open. Keep them with peaceful tankmates of their own scale or alone as a species group; large or boisterous fish turn them back into shadows, however good the ratio is.
Reading whether the group is right
A settled group tells you so. The males colour up, hold their small territories among the plants, and circle-display at each other through the day; females cruise and graze; the whole group scatters at a shadow and regroups within minutes. Constant hiding, pale fish, one female looking thin and harassed, or a male sulking in a corner all say the balance is off - usually too few females, too little cover, or too few fish overall. Adjust the group before adjusting anything else. For a nano tank stocking list that needs its numbers checked, put a follow-up question to KinsLe's Kinny; he answers using the KinsLe Answers library.