The short answer for young white clouds
Young white cloud mountain minnows can move into the main tank when they are too large to be eaten by the adults, swim strongly, and compete at feeding time. Size alone is not the whole test, because a long but still thread-thin juvenile can be chased and outcompeted even if it escapes being swallowed. Look for a deeper body, confident schooling with its siblings, and the ability to hold position in a gentle current without being pushed into corners or plant thickets.
Why adult white clouds are a risk to fry
Adult white cloud mountain minnows are peaceful toward fish their own size, but they treat very small fry as live food, because in the wild tiny moving specks near the surface usually are food. A juvenile that still looks translucent, with a narrow belly and undeveloped colour, reads as prey to an adult that would ignore a chunkier youngster. The grow-out period is therefore less about a birthday and more about crossing the size and sturdiness gap between fry and fish in the adults' eyes.
Signs a young minnow is ready to move
A ready young white cloud shows several signs together. Its body has filled out so the belly is no longer a thin line, its fins carry the beginnings of adult colour, and it darts to food at the surface instead of waiting for scraps to sink. It should also handle the main tank's filter outflow during a trial swim without tiring quickly. If one juvenile in the group lags far behind the others in size, hold that one back, because mixed moves leave the smallest fish carrying all the risk.
Moving day, step by step
Acclimate the young white clouds gradually, because grow-out water and main-tank water can differ in temperature and chemistry even in the same home. Float or drip-mix over a patient spell, then release the juveniles near dense planting so they have an immediate refuge while they map the new space. Add them when the lights are low and the adults have just been fed, since full, dimmed adults investigate newcomers less. Feed the tank lightly at first and watch that the youngsters actually eat rather than hovering at the edges. Transfer the juveniles in a soft net or, better, a small submerged container, because young minnows lifted in a net can damage delicate fin edges and arrive in their new home already stressed. Release them in a small group rather than one by one, since a lone juvenile is a target for curiosity while a little school of newcomers reads as part of the scenery within minutes.
Watching the first days after the move
Watch the young white clouds closely for the first several days. Chasing that ends in the plants is normal settling behaviour, but a juvenile that stops feeding, clamps its fins, or hides all day is telling you the move came early, and returning it to the grow-out tank is the kind move. Check that no youngster is trapped behind the filter intake, a real hazard for small fish in a stronger flow. Note which juveniles moved and which stayed behind so the next check compares like with like. Recording those move notes and feeding observations in KinsLe Care Teams lets every helper see which young minnows transferred, which stayed in grow-out, and what to watch at the next feed.