Safety at one inch long
A chili rasbora is one of the smallest fish in the hobby - an adult barely reaches an inch - and almost everything in its native peat-swamp streams is bigger than it is. Its answer to being permanently edible is the shoal: a loose cloud of identical red specks that confuses a predator's aim and puts dozens of eyes on watch at once. That instinct does not switch off in an aquarium. A chili rasbora judges its safety by the number of red bodies around it, and it behaves accordingly, which is why group size is the first and most important decision you make for this species.
Eight to ten is the real minimum
You will sometimes see six quoted as a minimum for chili rasboras, but eight to ten is where their behaviour actually changes, and more is better still if the tank is calm. In a proper shoal, the fish hold a deep brick-red, the males brightest of all, and the group drifts and feeds in the open water in the relaxed, wandering way that makes them worth keeping. In a group of three or four, the same fish go pale, flatten themselves against the planting, and may not come out even for food. The difference is so stark that keepers regularly mistake a too-small group for sick fish. If you are choosing between six chilis and ten, choose ten every time; the tank will look fuller and the fish will act like different animals.
The tank that lets a big shoal work
A large shoal of chili rasboras does not need a large tank, because their combined body mass is tiny and their waste output minimal - a modest nano aquarium houses a group of ten comfortably. What the tank does need is calm: a gentle filter outflow rather than a current to fight, dark substrate and background that set off the red and mimic their tannin-stained home water, and plenty of plants to drift among. Tankmates must be chosen on the same gentle scale - other tiny, peaceful fish, shrimp, or nobody at all. One boisterous fish in the tank can undo the security a shoal of twelve provides.
Feeding a shoal of tiny mouths
Group size connects to feeding in a way keepers miss: in a proper shoal, chili rasboras compete just enough to feed boldly, sweeping up small foods as a group. Their mouths are minute, so offer appropriately small fare - crushed fine foods and tiny live or frozen offerings - spread across the surface and mid-water so the whole shoal eats, not just the front row. In too small a group, the fish feed timidly and the shyest may get nothing, which is another reason undersized groups fade. A settled shoal feeding together in the open is the clearest sign the numbers are right. Under the heading of Family Sharing, KinsLe lets an owner open profile access for the rasbora group to particular family members.