Rasbora
Rasboras are what we recommend when someone wants the look of a tetra school in a smaller or gentler tank. They are hardy, peaceful, undemanding, and most of them stay small.
Chili rasboras and the nano tank
Chili (mosquito) rasboras top out around three quarters of an inch, which makes them one of the few schooling fish genuinely suited to a small planted tank. They need warmth, quiet tank mates and a well-planted scape to feel secure — in a bare bright tank they stay pale and hide. Against dark substrate with cover, a group of fifteen or twenty is one of the best-looking things you can put in a nano.
Harlequins are the reliable one
If you want a school that simply works, harlequin rasboras are hard to beat: about two inches, unbothered by a wide range of conditions including our hard alkaline tap water, and they hold a tight visible school rather than scattering. They are a better first schooling fish than neon tetras, which are more sensitive than their reputation suggests.
Numbers, again
Same rule as every shoaling fish: buy more of one species rather than a few each of several. Rasboras kept in small groups hide and lose color. Ten or more of one kind will behave normally and look like what you saw in the photo that made you want them.