The short answer for a threadfin rainbowfish
Plan a 20-gallon long tank as the starting point for threadfin rainbowfish. The fish are small and slim, but males trail extraordinarily long, fine extensions on their dorsal and anal fins, and a school spends the day in slow display, males raising those fins like sails as they circle females and each other. A short tank truncates every display flight and crowds the finnage into nipping range of tankmates. Length, calm water, and a school of eight to ten or more let the behaviour this species is kept for unfold at its natural, unhurried pace.
Why gentle flow belongs in the sizing decision
Threadfins come from slow, clear, heavily vegetated waters in New Guinea and northern Australia, and their extended fins are built for stillness, not current. A powerful filter outflow in a small tank forces them to swim constantly just to hold position, and the trailing fins fray and fold. In the 20-gallon long, place flow at one end and planting at the other so fish can choose moving or still water, and aim returns along the glass rather than across the display lane. That gradient is a luxury a nano tank cannot offer, and it is a genuine part of why the bigger footprint suits them.
Their mouths are small and upturned relative to their size, and they feed delicately at the surface and midwater. Any tank plan must include small foods offered where the school hovers, because they will not compete successfully at a busy community feeding station.
Planting the edges, opening the stage
Set tall stems and fine-leaved plants along the back and ends, float plants to soften the light, and keep the central water open as a stage. Males display most in soft morning-style light over that open lane, fins spread against the darker planting behind. A snug lid is essential, as rainbowfish as a family jump when startled, and a displaying male chasing across the surface can leave through an open feeding hatch. Choose companions from the smallest, calmest species only, or keep a species school; fin-nippers are disqualifying tankmates for fish whose whole beauty is trailing finnage, and larger fish regard them as snacks.
Numbers matter to fin safety as well as confidence: in a full school, male display attention distributes across many fish, while a lone male in a small group harasses the few females without pause.
Signs a threadfin school needs more space
Folded or fraying fins, males displaying only in one cramped corner, females hiding full-time in the stems, and fish pinned at the calm end away from the outflow all indicate the enclosure or its flow is undersized for the school. Correct the flow first, then length, before blaming the fish. Given calm length and company, threadfins are one of the most graceful sights in a planted tank. Recording the school count, the baffled filter setting, and their small-food routine in KinsLe Family Sharing ensures anyone covering care keeps the current gentle and the display school intact.