Time the move by her shape, not the calendar
A guppy pregnancy runs about a month, but moving her too early is the commonest mistake: days of confinement stress her, and stressed females can drop fry prematurely or hold them too long. Watch for the reliable late signs instead. Her belly squares off — fishkeepers call it looking boxy — when viewed from above and the front, and the dark gravid spot near her tail grows large and very dark. She may also go quiet, hovering near plants or the heater. That combination means birth is close, usually within a day or two, and that is the moment to move her.
Choose a planted nursery over a cramped trap
The small slotted breeder boxes that hang inside the main tank protect fry, but many females find them stressful, and a panicked guppy can injure herself against the sides. A better setup is a small separate tank — or a large, roomy nursery — filled with water from her home tank and packed with plants: floating plants at the surface, a loose mass of java moss or guppy grass below. The plants do the real work of this whole method. Guppy fry instinctively dive into cover the second they are born, and dense greenery gives them somewhere the mother simply cannot follow.
Move her gently and keep everything familiar
Net her slowly, or better, coax her into a submerged container so she never leaves the water and is never squeezed by a net around her swollen belly. Set the nursery somewhere quiet with the same temperature as the main tank, a gentle sponge filter or none at all for a short stay, and subdued light. Feed her a good meal once she settles — a well-fed mother is slower to hunt — and then leave her alone. Constant peering and tapping keep her stressed, and stress delays the birth you are waiting for.
Take her out the moment she is done
Guppies give birth over several hours, a few fry at a time, and the mother's appetite returns quickly. The clue that she has finished is her shape: the boxy belly is gone, the gravid spot has shrunk, and she starts actively nosing through the plants. That is your cue to net her back to the main tank promptly and feed her there. Every extra hour she spends in the nursery costs fry. If you cannot watch continuously, check at least every couple of hours once births start, and remove her at the first check where she looks slim again.
Raise the fry where they were born
Leave the fry in the planted nursery rather than moving them again. Feed powdered fry food or crushed flake several times a day in pinches, siphon the bottom gently, and let the plants keep working as cover and as grazing surfaces that grow microscopic food. Within a few weeks the fry are big enough that no adult guppy can swallow them, and they can join the main tank. Because the critical job is catching the short window when the birth finishes, splitting those checks between family members through KinsLe Care Teams — each look-in recorded with a timestamp — means the mother is never left hunting through the moss unnoticed for half a day.