What the box actually does
The box itself is simple: a small slotted or meshed plastic container that clips inside the main aquarium. A pregnant livebearer goes in it, gives birth there, and the fry drop through a grate or are sheltered in a lower chamber where the mother cannot reach them. Because the slots let tank water circulate through, the fry get heated, filtered water at the main tank's temperature with no extra equipment. Used that way — as a birthing ward for a day or two — the box is genuinely useful, cheap, and always ready in a drawer.
The first limit: space, and what it does to growth
Everything about the box is small, and fry feel that quickly. A brood of guppy fry can spend their first days in one, but within a week or two the crowding shows: growth slows, the runts fall further behind, and waste builds up in a volume of water barely bigger than a mug. Fry raised in a separate grow-out tank, with room to swim and spread out at feeding, typically grow faster and more evenly than the same brood kept in a box. The box buys safety; it does not provide a childhood.
The second limit: the mother's stress
The box confines the adult as much as it protects the young. Many females pace the walls of a breeder box, refuse food, or injure their mouths pushing at the plastic, and a badly stressed guppy can delay her birth or drop the fry early. Roomy mesh nursery bags and boxes with a plant tucked inside reduce this, but for a nervous female a separate planted birthing tank is kinder. Watch her behaviour honestly: if she is frantic after an hour in the box, the method is costing more than it saves.
The third limit: shared water means shared problems
Because the box breathes the main tank's water, it inherits everything in that water. If the main tank's filter slips and ammonia rises, the fry in the box get the worst of it, trapped in the smallest volume. If disease breaks out among the adults, the fry are exposed too — the box is a fence, not a quarantine. Food is another shared-water problem: powdered fry food fed into the box mostly drifts out through the slots and fouls the main tank, while the fry get what happens to stay. A separate tank has its own sponge filter, its own water you control, and food that lands where the fry are.
Choosing between them for your brood
The practical rule most keepers land on: use the box for the emergency and the short stay — an unexpected birth tonight, a few days until fry are too big to swallow — and use a separate tank whenever you actually intend to raise a brood to a good size. The separate tank need not be grand; a modest bare tank with a sponge filter, a moss clump and a heater outperforms any box within a fortnight. If egg layers or a large brood shift that balance, put a follow-up question to KinsLe’s Kinny; he answers using the KinsLe Answers library.