Judging size for a surface-dwelling fish
Young clown killifish are ready for the main tank when their length and body depth clearly exceed what an adult killifish could attempt to eat. Clown killifish are small, but adults will take surprisingly small tankmates that drift near the surface, and the youngest fish spend their lives in exactly that same layer. Compare juveniles with the adults that will receive them and favour patience, because these fish grow steadily and a short extra spell in the rearing tank carries little cost. A ready juvenile looks complete in miniature: striped pattern visible, fins intact, body filled out from regular feeding, and an active interest in food at the surface. Thin, still-translucent youngsters should stay where meals are guaranteed and competition is absent.
A lid with no gaps is non-negotiable
In young clown killifish, housing security matters as much as size. This species is an accomplished jumper that finds gaps around filter cables, feeding hatches, and warped lid corners, and young fish startle more easily than settled adults during their first days in a new tank. Before any move, seal the receiving tank completely, check every cut-out, and keep the water level and cover arrangement the same as the fish are used to. Floating plants across much of the surface calm surface fish, give youngsters immediate refuge from adult attention, and reduce the urge to leap at shadows. A tank that is almost covered is, for a killifish, simply an uncovered tank with a smaller exit, so treat the lid check as a pass-or-wait decision.
Feeding at the surface alongside adults
Young clown killifish should already be taking the foods the main tank receives before they move, because their upturned mouths are built for picking small items from the surface film and slow learners miss out among quicker adults. Confirm in the rearing tank that every juvenile rises promptly and eats its share of fine floating foods and tiny live foods, rather than hanging below while bolder fish feed. On moving day, add several youngsters together, dim the lights, and offer a small surface feed shortly after release so the group's first experience of the tank centres on eating rather than being inspected. Spread that first feed widely across the floating plants so each newcomer finds food in its own patch of cover.
Watching the surface layer settle
For young clown killifish newly placed with adults, judge the move by behaviour in the top layer: youngsters weaving through floating roots, feeding at the film, and resting in cover between meals are settling well. One that stays pinned in a corner, is repeatedly darted at, or never reaches a feed needs more growing time in the rearing tank. Count the group at the surface each day for the first week, because a missing killifish is more likely to have jumped through a missed gap than to be hiding for days. Keep notes brief but specific: who feeds, who shelters, and whether the count stays complete. Listing the lid check, release, and daily surface counts in KinsLe through Care Teams gives every helper the same settling picture, so a missing or bullied youngster triggers action the same day.