The mouth test decides most of it
A breeder net protects fry from exactly one thing: being eaten. So the first readiness sign is a comparison, not a measurement — hold the fry's size up against the largest mouth among the adults it will join. Ready means plainly too big to be swallowed, with depth of body to spare, not a near thing you are hoping about. Predation in a community tank is quick and usually happens in the first hour, often from the one fish you underestimated. If any adult in the tank could plausibly manage the fry, the answer is not yet, whatever the fry's age; fast-growing batches tempt early releases, and the calendar is the worst guide you own. Size up the smallest fry you plan to release, not the biggest, because the release decision applies to each fish individually.
Swimming strength is the second test
Size alone is not enough if the fry cannot cope with open water. In the net, look for fry that hold their position against the gentle pull of the filter, swim the full length of the net rather than drifting, and strike at food the moment it lands. Fry that still hug the mesh, startle at shadows passing the glass, or get pushed around by the current are telling you their muscles and confidence are not there yet. Out in the tank they will need to cross open ground to reach cover and compete at feeding time against adults who have done it ten thousand times; a fry that cannot manage the net's mild conditions will not manage that. This test also catches the quiet runts in a batch whose size looked acceptable but whose strength did not.
The miniature-adult look confirms it
By release readiness, a fry should look like a scaled-down adult rather than a translucent sliver: fins fully formed and carried open, body filled out with real depth, colour and pattern showing, and that glassy, see-through quality of the newborn stage gone. Development you can see tracks development you cannot — digestion able to handle the main tank's foods, senses sharp enough to read tankmates. Release in stages if the batch is uneven: the largest, strongest third can go first while the rest grow on in the net for another week, and staggered releases lose fewer fish than one all-or-nothing day. Breeder nets also crowd and foul faster as fry grow, so a batch that has clearly outgrown the net's water quality is giving you a sign of its own.
Release gently, and watch the first hour
Float the net in the main tank briefly so temperatures match, then let the fry swim out on their own near plants or decor that give instant cover, ideally just before a feed distracts the adults. Stay and watch the first hour: occasional curiosity from adults is normal, but repeated chasing means the release was early, and the fry go back to the net without debate. Most well-judged releases are undramatic to the point of anticlimax, which is exactly the outcome you are aiming for. Recording the release alongside the batch's hatch details in your KinsLe Pet Passport — sizes, numbers and how they settled — turns one batch's experience into guidance for the next spawn.