Size against adult mouths is the first gate
A young tiger barb is ready for the main tank when its body is deep and large enough that an adult barb, or any larger fish in that tank, cannot simply eat it, because tiger barb fry start small enough to be treated as live food. Judge by the biggest mouth in the receiving tank rather than by the youngsters' age, since growth rates vary widely between batches. The young fish should also show the clear dark striping and rounded body of a juvenile barb rather than the transparent, thread-like look of a new fry. If an adult could still fit the youngster in its mouth, the nursery remains the right address.
Schooling strength matters as much as size
Tiger barbs are schooling fish with a nippy streak, so a young tiger barb joining adults needs the swimming strength and confidence to stay with a fast, pushy group. A youngster that feeds at the surface alongside siblings, darts without tiring, and holds formation when the school turns will cope with adult energy far better than a lingering fish of similar size. Moving several young barbs together gives newcomers their own sub-school to shelter inside, which spreads adult attention and reduces the picking-on that a lone small fish attracts. A single young barb added alone to settled adults is the hardest version of this move, whatever its size.
Preparing the main tank for newcomers
A main tank ready for young tiger barbs offers plant cover at mid-water where youngsters can break line of sight, open swimming space for the school, and no slow, long-finned tankmates whose fins would invite the newcomers' nipping as they grow. Check the receiving group itself too, because a very small adult school can be more aggressive toward additions than a larger, settled one. Feed the adults well shortly before the release, dim the lights if you can, and avoid adding youngsters on the same day as new decorations or other new fish. Several changes at once make it impossible to tell what unsettled the tank if trouble follows.
Watching the school over the first week
A young tiger barb settling into a main tank joins the school's rhythm within days, feeding at the surface rush, resting in the group's loose formation, and showing clean fins and steady colour. One that hides alone continuously, is chased from every feeding, or develops nipped, ragged fins is not integrating and may need to return to the nursery to grow on. Brief chases and jostling are ordinary tiger barb politics and should not trigger a rescue on their own. Count the youngsters at feeding time daily for the first stretch, since that is the one moment the whole school is visible at once.
Making the group move a shared job
A barb move involves preparation, a timed release, and a week of feeding-time counts, and it goes better when those jobs are divided rather than assumed. One person prepares cover and feeds the adults, another releases the youngsters, and the counts get noted so a missing fish is spotted on the day it happens rather than at the weekend clean. Record how many moved and how the school behaved, because the next batch from the nursery will follow the same path. Setting up that checklist in KinsLe through Care Teams assigns each step to a helper and stamps it done when finished.