The mouth-size rule for young guppies
Young guppies can join adults once they no longer fit in an adult guppy's mouth, and that threshold is about body depth and sturdiness rather than length alone. A newborn guppy is a slender thread with eyes, and adults — including its own mother — will eat fry for the first stretch of life. By the time a young guppy has filled out, carries a rounded belly profile, and swims with a confident wag rather than a frantic dart, adults generally leave it alone. Even then, fine-leaved plants, floating cover, and moss in the main tank give young guppies escape routes during the first days, because a determined adult may still chase a youngster that looks borderline. If you cannot tell at a glance whether a particular fry is safely past bite size, it is not yet.
Sexing changes the moving plan for guppies
Young guppies bring a decision most fry do not: their sexes become distinguishable young, with males developing colour and a pointed anal fin while females stay larger and plainer with a rounded fin. If the main tank is a mixed adult community and you are content for breeding to continue, youngsters can move as one group once they pass the size test. If you are managing numbers, line-breeding, or a females-only display, sort the young guppies as they are moved and house males separately from that point onward. Waiting too long to separate defeats the purpose, because young female guppies can store sperm and produce fry long after a single contact with males. Decide the plan before moving day, not while the net is already in the water.
Acclimating young guppies to the adult tank
Young guppies raised in a fry box, breeding trap, or grow-out tank need gradual acclimation to the main tank's temperature and water, especially if the fry setup has been kept warmer to speed growth. Release them among plants rather than in open water, and feed the adults at the far end of the tank at the same moment so attention is elsewhere. Young guppies that have lived in a small fry container may startle at open space and larger tankmates, so subdued light for the first hour helps them find cover calmly. Check heaters, filter intakes, and any gaps at the hood, because small young guppies explore relentlessly and a fry that slips into a filter compartment rarely thrives there.
Watching the youngsters in their first week with adults
A young guppy that has settled feeds alongside the adults, holds a place in the group without constant fleeing, and keeps its tail fin spread rather than clamped. Chasing by adult males toward young females is expected social behaviour, but a youngster hiding continuously, missing every feed, or showing nipped tail edges needs to go back to grow on. Count the youngsters daily at feeding for the first week, because losses among newly moved guppies are easy to miss in a planted tank. Note which colour lines and sexes moved, and when, in KinsLe through Care Teams so the next person feeding does not add a mixed handful of fry to the wrong tank.