Size against adult mouths comes first
Young boesemani rainbowfish are ready for the main tank when their body depth and length put them safely beyond what an adult rainbowfish can swallow. Adult boesemani are large, active fish with generous mouths, and a juvenile that still looks like a snack-sized sliver in the rearing tank will be chased, even by otherwise peaceful schoolmates. Compare the young fish directly with the smallest adult's mouth rather than relying on age, because broods raised at different temperatures and feeding levels grow at very different rates. The young fish should also look filled out rather than narrow, with a body that has begun to deepen toward the adult rainbow shape. If you are unsure, waiting until the juveniles grow a little more costs nothing, while moving early can cost the whole brood.
Swimming strength for a livelier tank
In young boesemani rainbowfish, readiness also shows in how they handle moving water and open space. The main tank usually has stronger filtration, longer swimming runs, and adults that surge at feeding time, so a juvenile that still tires quickly or shelters constantly in the rearing tank is not ready for that pace. Watch for young fish that cruise the whole rearing tank, recover instantly after darting, and hold position in the filter flow without being pushed backward. Those are the fish that will keep up when the adult school sweeps across the tank at a feed. Fish that still hug plants and startle at every shadow benefit from more growing time and denser cover before facing a busier environment.
Eating the adult menu without help
Young boesemani rainbowfish should move only once they are taking the same range of foods offered to adults, at a size they can manage without special preparation. In the main tank there is no separate fry feeding, and slow or picky juveniles lose out quickly when faster adults gather. Test readiness in the rearing tank by feeding the adult menu for several days and confirming every juvenile eats promptly and keeps condition, rather than a few bold fish monopolising each meal. Move the young fish in a group of several at once, ideally near a water change and with lights low, so no single newcomer carries the school's attention, and add them when you can watch the first feeding that follows.
The first days after the move
For young boesemani rainbowfish newly placed with adults, the settling signs are relaxed schooling, normal colour for juveniles, and confident arrival at feeds alongside larger fish. Some initial wariness and time spent near plants is expected, but persistent hiding, torn fins, or juveniles being driven from every meal means the move came too early and they should return to the rearing tank to grow on. Keep a brief note of who eats, who hides, and whether any young fish loses condition during the first week, because a slow decline is easy to miss in a busy display tank. Check water quality after adding a group, since the extra fish raise the load on the filter. Planning the size check, group move, and first-week feeding watch in KinsLe through Care Teams keeps each step owned by someone, so a struggling juvenile is caught and moved back promptly.