Think in Millimetres, Not Inches
A chili rasbora is fully grown at well under an inch, with a mouth sized for specks. That single fact writes the tank mate rules: any companion must be small enough not to eat a chili, slow enough not to strip every meal before the school arrives, and calm enough not to send these shy fish into permanent hiding. Nano communities are built from a short list of genuinely tiny species, and the discipline is resisting the much longer list of small-looking juveniles that will grow into a chili's predator within a few months.
Companions That Share a Nano Tank Well
Pygmy corydoras are the classic floor partner, small enough to school beneath the chilis without disturbing them. Other micro rasboras of similar size, ember-sized tetras, and tiny danio species can share the mid-water if the tank is large enough for two schools to keep their distance. Otocinclus handle algae on plant leaves without bothering anyone, and dwarf shrimp and small snails complete a nano setup — with the understanding that adult shrimp are safe companions while their tiniest babies may be picked off by any fish, chilis included. A single calm centrepiece such as a sparkling gourami can work, but only in a bigger nano tank and with close watching.
Fish That Do Not Belong With Chilis
Anything with a mouth a chili fits into is out: most tetras beyond the ember size, all but the smallest barbs, bettas with a hunting temperament, larger gouramis, angelfish, and community staples like platies once grown. Fast, bold feeders such as zebra danios in a small tank cause a quieter problem — the chilis simply never reach the food and fade over weeks without a single act of aggression to witness. Even a well-meaning addition like a juvenile bristlenose grows into a bulldozer among nano plants and fish. When in doubt, compare adult sizes, not the size on the shop label.
Setup Choices That Make Mates Compatible
Compatibility is partly the tank itself. Dense fine-leaved planting, dark substrate, and dim or plant-shaded light make chilis bold enough to school in the open and give them instant refuge when a companion startles them. Keep the chili school itself at ten or more so its confidence does not depend on tank mates, feed crushed micro-food in small amounts where the school gathers, and run gentle filtration that cannot trap fish the size of a grain of rice against an intake. Stable, soft, slightly warm water suits the whole nano cast, which is one reason the approved list is short.
Watching a School You Can Barely See
Chilis announce trouble by disappearing: a healthy school is out, coloured a deep red in the males, and feeding in the open, while a stressed school pales and vanishes into the plants for days. Count the school at feeding time, note any fish that stops joining, and suspect new tank mates first when behaviour changes after a stocking. Because these fish are so small, one person's observation — a gourami seen lunging, a school one short — is easy for another keeper in the house to miss entirely. KinsLe Family Sharing keeps the nano tank's profile and care notes shared, so every family member spots the same changes.