Schooling behaviour is the readiness signal for rasboras
A young harlequin rasbora is ready for the main tank when it behaves like a member of a school: holding position with its siblings in open water, turning together, and coming forward as a group when food appears. Harlequin rasboras are defined by that schooling habit, and a youngster still hiding alone in plants is telling you it feels vulnerable, whatever its size. Colour backs up the judgement — the species' coppery body tone and bold dark wedge along the rear half deepen as a young harlequin rasbora grows, and a pale, washed-out youngster that has not coloured up beside its siblings may be stressed or underfed in the rearing tank. Adult harlequin rasboras will accept youngsters that school with them, often absorbing them into the group within hours.
Judge the whole main tank, not just the adult rasboras
Adult harlequin rasboras pose no threat to their young, but the main tank's other residents decide the safe size. A large gourami, angelfish, or other wide-mouthed community fish can take a small rasbora, so young harlequin rasboras should be stout enough to be ignored by the biggest mouth in that specific tank. If the main tank holds only small peaceful species, youngsters can move earlier than they could into a tank with predators, which is why no single length suits every setup. Also check current: harlequin rasboras come from gentle waters, and a young fish fighting a strong filter outflow all day arrives at feeding already tired. Baffle harsh flow before the move rather than after a youngster fails to settle.
Moving a young school without breaking it up
Move young harlequin rasboras as one school, acclimating the whole group together so they enter the main tank with their social structure intact. Catching schooling fry one by one with a net stresses each fish repeatedly; herding the group gently into a submerged container keeps the young harlequin rasboras calmer and preserves the school. Release them near the adults' preferred area, often mid-water beneath floating plants, so the two groups merge naturally instead of the youngsters claiming a distant corner. Feed lightly soon after release and watch whether the youngsters rise with the adults or hang back below them. A merged school that feeds as one within the first day is the clearest sign the timing was right.
After the move, watch the edges of the school
In an established main tank, check young harlequin rasboras at the school's edges, because the smallest youngsters get pushed to the fringe where they feed last and rest apart. A youngster that repeatedly drifts alone, loses its copper colour, or shows a pinched belly needs closer feeding attention or a return to the rearing tank. Keep the group's total numbers healthy — harlequin rasboras school more confidently in a larger group, and losing a few youngsters unnoticed can leave the remainder skittish. Note the school's count, the moving date, and any youngster returned to grow on in KinsLe through Family Sharing so family members doing headcounts at feeding time know the number they should expect to see.