The short answer for a displaying school
Threadfin rainbowfish are slim, delicate fish whose males carry fins so long and trailing that they seem built for a bigger body. A school belongs in a tank of at least 20 gallons with a long footprint, and 30 gallons gives their behaviour proper room. These are mid-water fish that spend the day in motion: males flare their thread-like fins at rivals and females, the school drifts, breaks, and reforms, and everyone retreats into planting when startled. A short tank truncates the display runs and keeps the group bunched; a long one lets you watch the full performance the species is kept for.
Space for fins, not just bodies
Most stocking advice measures a fish by its length, but threadfins must be measured by their finnage and their manners. Those trailing fins tear easily and heal slowly, so the tank needs open water free of sharp decor, rough rock edges, and nippy companions, plus enough width that displaying males are not constantly fin-to-fin. Males are competitive show-offs rather than fighters, yet in cramped quarters the constant close-range displaying frays both fins and nerves, and subordinate males stop feeding well. Planted margins matter just as much as the open centre: females and off-duty males rest among fine-leaved stems, and the whole school needs that green refuge within a quick dash of the display ground.
Setting up a gentle, long tank
Choose length over height, plant the back and ends with soft, fine-leaved plants, and keep a broad open lane through the middle and front. Flow should be mild and diffuse, because a strong current streams those long fins, exhausts the fish, and turns their display ground into a wind tunnel. A tight lid is essential, since threadfins are startled jumpers, and any gap around cables should be blocked. Soft light over the open lane shows the males' colours and fin extensions at their best, while shaded planting at the ends gives the school a gradient to settle into at night. Keep the waterline a little lower if your lid is loose, and net fish slowly during maintenance to avoid snagging fins. Offer their small foods in the calm edge of the display lane, where threadfins can pick delicacies from the water without competing against faster fish or fighting the current for every bite.
Making the group work in the space
Keep threadfins in a proper school, with more females than males, so display energy is spread around and no single female is courted relentlessly. In a 20-gallon tank a modest group works; the larger tank allows a bigger school, which usually means calmer fish and better colour. Companions must be small and peaceful, since anything boisterous will outcompete these slow, delicate feeders and anything nippy will shred the fins you bought the fish for. A settled school displays in the open, feeds delicately but eagerly at the surface and mid-water, and rests spread through the planting. Torn fins, males hiding singly, or a school that never leaves the plants points to crowding, current, or company, in that order. Families tending these rainbows together can rely on KinsLe's Family Sharing to keep the school's ratio, tank length, and feeding notes visible to everyone who helps.