Keeps fry safe without a second tank.
It hangs inside the main tank, so the fry are in water that is already heated, already cycled and already the right chemistry. That is the whole advantage over a separate container.
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Why you'll love it
The usual advice is a second heated, cycled tank, which almost nobody has ready when a fish drops fry unexpectedly. A box hanging in the main tank borrows the water you already have.
A sealed tub goes stale and swings in temperature within hours. Slotted sides keep the box on the same water as the tank, so the parameters cannot drift away from the fish inside.
Half of what this gets used for is not breeding. Isolating a nipped or bloated fish where you can watch it, and dose it, without netting it out of the system.
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A separate container needs its own heater, its own filter and its own cycle, and setting one up in a hurry usually means none of the three. Fry moved to a bucket of fresh water face a temperature change, no biological filtration and rising ammonia within a day, which is worse than the risk they were moved away from.
A box hanging in the main tank sidesteps all of it. Slots let the tank water pass through, so temperature, chemistry and filtration are shared with the display. Nothing has to be matched because nothing is different, and there is no acclimatisation needed going in or coming out.
Livebearers eat their own young, often within minutes. The divider addresses this by letting a heavily pregnant female sit above while the fry drop through to the section below, which puts them out of reach immediately without you having to catch anything.
Remove the female as soon as she has finished. Leaving her confined for days is stressful and she will not deliver well under it, and the box is small. The fry can then stay for a week or two before they need more room, which is well before they outgrow it.
Check the slot width against the size of your fry before trusting it. Very small fry from egg layers can pass through slots designed for livebearer fry, and the first you know is an empty box. If in doubt, line the inside with a fine mesh for the first week.
Water exchange depends on flow reaching the box, so hang it where there is gentle circulation rather than in a dead corner. In a still spot the water inside stagnates despite the slots, and a small air stone or positioning it nearer the filter outflow solves that without creating a current the fry cannot handle.
In the box
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The details
Loved by real pet parents
My platy dropped fry with no warning and I had nothing ready, which is apparently the standard experience. This hangs in the main tank on its suction cups so the fry got the heated cycled water I already had. Twelve of them grew out in it and went into the tank proper three weeks later.
I had tried a floating plastic tub before and the water inside went stale within a day. Because the sides are slotted, the water in this is the same water as the tank, so the temperature and the readings never drift. That is the difference between a breeding box and a bucket.
It does everything described but the suction cups will not hold on glass with any algae film on it. I wipe the spot with a blade first and then they stay put for weeks. Once mounted the box does not sag, even with the divider in and a lid on top.
I bought it expecting to breed and instead it has mostly been where a nipped fish goes to be left alone for a few days. Being able to watch one closely without netting it out of the system has been genuinely useful. The vented lid stops anything jumping out while it recovers.
The slotted divider keeps the mother on one side and the fry drop through to the other, which is exactly what you need with guppies. Without it she would have eaten most of them within the hour. Everything clips together with no tools and comes apart for cleaning.
At about 12.5 by 7 by 7 cm it is fine for a dozen fry or one isolated adult, but I had a much larger batch than that and it was tight. I ended up running it in shifts and moving the biggest ones out early. Well made for what it is, just measure your expectations.
The clear sides let me count the fry every morning without disturbing anything, which mattered in the first week. It sits high enough on the glass that the slots stay under the surface and the lid stays above it. Nothing has yellowed after five months, though the plastic does scuff if you clean it with anything rougher than a soft cloth.
I use it to hold shrimplets I want to keep separate before they go into a grow out tank. The slots are fine enough that nothing has escaped through them and the flow still gets in. Comes apart into four pieces for a rinse, which takes about a minute.
Mine sits very slightly higher than I would like, so the top row of slots is above the water line and only the lower ones do anything. It still keeps the water exchanging and the fry were fine. I would just have preferred the mounting to let me drop it another centimetre.
Good questions
The slots are sized to hold newborn livebearer fry. Very small egg layer fry are another matter. For those, keep the divider in and watch the first day before you trust it.
Days, not weeks. It is a nursery and a hospital, not accommodation. Fry go back out as soon as they are too big to be eaten.
Not usually, since tank water moves through the slots. If you have a very still tank, park it near the filter outlet so there is some flow past it.
Very small fry from egg laying species can pass through. Check the slot width against your fry, and line the box with fine mesh for the first week if you are unsure.
Days rather than weeks for an adult, since it is small and confinement is stressful. Fry can stay a week or two before they need more space.
Not if it hangs where there is gentle circulation. In a still corner the water inside stagnates despite the slots, and a small air stone or a better position fixes it.
Last reviewed 30 August 2026