Beyond prey size, into tankmate size
Young scarlet badis are ready for the main tank when they have grown clearly past the size adult badis and other small predators treat as live food. Adult scarlet badis hunt tiny creatures among plants all day, and a very young juvenile drifting through a male's territory can be taken without any aggression being involved. Judge readiness by comparing body length and depth with the adults' mouths and by how complete the young fish look: defined fins, steady hovering, and confident short darts between cover all signal a fish that reads as a resident rather than a morsel. Colour is not the test, because young males colour slowly and females stay muted, so a pale youngster can still be perfectly ready if its size and behaviour are right.
Self-sufficient on live foods first
In young scarlet badis, feeding independence is the second gate for the move. Many of these fish feed best on small live foods and pick deliberately, one item at a time, which puts them at a disadvantage anywhere food arrives briefly and competition is quick. Before moving, confirm each youngster hunts and eats reliably in the rearing tank without food being delivered directly to its hiding place, and that its belly fills at most feeds. Plan how live food will reach the newcomers in the main tank, targeting their corners with a pipette-style delivery to the plants they occupy rather than broadcasting food the adults will clear first. A youngster that still waits for food to arrive should stay in rearing until its hunting is assured.
Territories, litter, and lines of sight
Young scarlet badis settle into adult tanks only where the planting lets them vanish and reappear on their own terms. Adult males each hold a small territory around favoured cover and display at rivals, so a sparse tank forces constant encounters that exhaust shy newcomers. Build the receiving tank with dense fine-leaved plants, leaf litter, and small caves that break sight lines into separate pockets, ideally more pockets than fish. Adding youngsters when plants have grown in, rather than on the day they are planted, gives them real depth of cover from the first hour. Females and young fish generally range more widely than territorial males, so watch especially that no single male can patrol every pocket the youngsters might use.
A quiet first week of observation
For young scarlet badis newly placed with adults, success is understated: a youngster holding a favourite thicket, emerging to hunt its corner, and retreating unhurriedly when an adult passes. Warning signs are a young fish hovering exposed at the surface, one chased through cover repeatedly, or a corner where food never gets taken. Check each known pocket at feeding time instead of searching the whole tank, and resist rearranging plants to find a fish, because your disturbance costs a shy species more than your uncertainty does. Keep a pocket-by-pocket note of sightings and feeds for the first week. Keeping that pocket map and feeding record in KinsLe through Care Teams means each helper checks the same shelters the same way, so a youngster that stops appearing is noticed within a day.