The calendar is a poor guide for moving fry into the main tank. A fast-growing batch from one spawning may be ready weeks before a slower batch of the same species, and the adults waiting in that tank do not care how old a fry is, only whether it fits in a mouth. The real question is whether each young fish has crossed three thresholds: too big to swallow, strong enough for the main tank's conditions, and bold enough to eat alongside adults.
The mouth-size test comes first
Most adult fish, including peaceful community species and even the fry's own parents, will eat any youngster that fits in their mouths. Judge the fry's body depth, not just its length: a deep-bodied youngster is safe sooner than a long, thin one of the same age. Hold the biggest adult in the main tank in mind as the measuring stick, because that fish sets the danger line. If you are unsure whether a fry clears it, it does not, and another week or two in the grow-out tank costs nothing compared with losing the batch in an afternoon.
Feeding competition is the second threshold
Surviving the adults is not the same as thriving with them. In the main tank, food arrives in a scramble, and fry raised on careful target feeding can be outcompeted by faster adults even when nobody is chasing them. Before the move, the youngsters should be eating the same foods the main tank receives, at the surface or midwater where the adults feed, rather than waiting for food to sink to them. Fry that still need tiny live foods or powdered meals several times a day belong in the grow-out setup, where that feeding schedule is possible, until they graduate to the adults' menu.
Match the water and the current before the move
Grow-out tanks are usually gentler places: sponge filters, still water, frequent small changes. The main tank may have stronger flow, deeper water, and a different temperature or chemistry if the two have drifted apart. Acclimate fry to the main tank's water gradually, the same way you would acclimate a newly bought fish, and check that the youngsters can hold position in the filter current without being pinned or exhausted. Moving day is also the wrong day for other disruptions, so avoid pairing the transfer with a major rescape or new adult additions.
Introduce a few, at feeding time, into cover
Move a small test group first rather than the whole batch. Release them during a normal feeding, when the adults are distracted, and make sure plants, caves, or other cover are already in place so the newcomers can vanish if chased. Then watch the first hour and the next feeding: brief interest from adults is normal, but persistent chasing, nipped fins, or fry hiding so completely they never eat means the test group goes back to grow-out for more growth. If the test group settles, feeds, and holds its own for a few days, the rest of the batch can follow.
Species habits shift the timing
Livebearer fry, born comparatively large, often reach the mouth-size line sooner than egg-layer fry that start life far smaller. Parents that guard eggs or fry may tolerate their own youngsters longer, while known fry-eaters in the community demand extra caution regardless of the parents. Bottom-dwelling fry face a different audience, since catfish and loaches patrol where they rest. Judge your actual tank, not a general chart. Keeping the size checks, test moves, and feeding watches from slipping is easier when KinsLe's Care Teams assigns each step for the fry to a named helper and stamps it complete.