The Fluval Hang-On Breeding Box sits outside the glass and holds about 1.1 litres, so fry drop through the V-separator without eating swimming room in the main tank. The Uniclife Acrylic Breeding Box is the in-tank alternative, a 17.3 x 10 x 10 cm floater with two dividers when you want the nursery inside the same water.
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These fish breeding boxes come from makers’ published specifications and live UK retail listings, checked October 2026. They have not been stocked or observed here. Sizes are the inner or outer figures the listing prints.
How Long Should a Fish Stay in a Breeding Box?
Treat every box as a short stay. A pregnant livebearer, a pair that is being spawned, or a bullied fish can rest there for hours or a few days, not as a second home. The box shares the tank water, so ammonia from extra food still hits the same filter. The OATA freshwater sheet lists companionship as one of the five legal welfare needs: some fish must be kept apart, and some must live in a group. A box can separate a fin-nipper overnight. It cannot replace a second cycled aquarium if two adults will never share space.
Move fry out as soon as they swim strongly and eat, or the mesh clogs and the adult starts to pace. If you are isolating a sick fish, test the main tank first. Poor water, not a missing hatchery incubator, is the usual cause of clamped fins. This page is only for fish breeding boxes. Nesting products for other animals live on the bird nest box and chicken nest box guides, and litter systems sit on the automatic litter box page.
What Size Isolation Box Fits the Adult and the Fry?
Measure the adult, then pick an isolation box that lets that fish turn without bending. The Fluval Hang-On Breeding Box is 16 cm wide and about 1.1 litres, hung on the rim, so the adult stays in a narrow slot while fry slip the V-plate. The Uniclife Acrylic Breeding Box and the Pawfly Acrylic Breeding Box both state an inner 17.3 x 10 x 10 cm with two removable separators that make three rooms. The Pawfly Large Fish Breeding Box steps up to 20.3 x 11.7 x 13.2 cm and six suction cups. The QWORK Aquarium Fish Breeding Box is 127 x 65 x 70 mm, which suits shrimp or newborn fry, not a full-grown guppy pair. The Plomveriq Acrylic Breeder Box is about 8 x 7 x 11 cm. Mesh crates run larger: the Jesiah Fish Breeding Box is 16 x 14.5 x 15.5 cm, the MDLSDE Fish Breeder Net is 16 x 14 x 15 cm and is stated for 35 to 150 litre tanks, and the IRYNA Fry Hatchery Net is 26 x 15 x 15 cm.
A box that is too small raises stress and waste. A box that is huge in a 40 litre tank steals swimming room from the rest of the stock. Leave a gap between the acrylic wall and the pane so a tail cannot wedge, which the Pawfly Large listing also flags with a double-sided cup.
Do Acrylic Boxes or Mesh Nets Exchange Water Better?
Mesh nets pass more flow and keep the water closer to the tank temperature, which matters if you rely on one heater in the main body. The Tabanzhe Fine Mesh Hatchery and the IRYNA Fry Hatchery Net use fine polyester or nylon so fry stay in while water moves. Acrylic boxes need side slots. The Uniclife and Pawfly acrylic models say those vents are enough without a second heater or air stone. The Fluval Hang-On Breeding Box is the exception: warm, oxygenated water is drawn through with an air pump that you must buy separately. If the pump stops, that 1.1 litres can go stale while the display tank still looks fine.
Clean mesh in tank water, not tap water, so you do not kill film bacteria and so chlorine does not sit on the net. Acrylic wipes with a soft cloth. Never use a steel algae blade on a clear box. Feed tiny portions of the same food the adults already take, because leftover crumbs in a 1 litre cell spike ammonia fast. The RSPCA environment page still wants zero ammonia and zero nitrite in the whole system, and a box does not get its own filter.
How Do These Fish Breeding Boxes Compare?
| Product | Type | Stated size | Fixing | Rooms or extras |
|---|---|---|---|---|
| Fluval Hang-On Breeding Box | External hang-on | 16 cm wide, 1.1 litres | Hangs on the rim | V-separator, needs an air pump |
| Uniclife Acrylic Breeding Box | Floating acrylic | Inner 17.3 x 10 x 10 cm | Two suction cups or float | Two dividers, three rooms |
| Pawfly Large Fish Breeding Box | Floating acrylic | 20.3 x 11.7 x 13.2 cm | Six suction cups or float | Two dividers, three rooms |
| Pawfly Acrylic Breeding Box | Floating acrylic | Inner 17.3 x 10 x 10 cm | Two suction cups or float | Two dividers, three rooms |
| QWORK Aquarium Fish Breeding Box | Clear PP | 127 x 65 x 70 mm | Suction cups, can float | Removable separators |
| Plomveriq Acrylic Breeder Box | Acrylic | About 8 x 7 x 11 cm | Suction cups, self-floating | Isolation plate included |
| Jesiah Fish Breeding Box | Nylon mesh | 16 x 14.5 x 15.5 cm | Four suction cups | Two dividers, 1 mm mesh |
| Tabanzhe Fine Mesh Hatchery | Fine mesh | Not stated as a single figure | Six suction cups | Fine mesh, feeding access |
| IRYNA Fry Hatchery Net | Polyester mesh | 26 x 15 x 15 cm | Six suction cups | Large hanging crate |
| MDLSDE Fish Breeder Net | Nylon mesh | 16 x 14 x 15 cm | Four suction cups | Two dividers, for 35 to 150 L tanks |
How Should You Install a Breeding Box Without Trapping Fins?
- Cycle and test the main tank first so ammonia and nitrite are already zero before you add a box.
- Rinse the box in tank water, fit the cups on clean glass, and check there is a gap so a tail cannot jam.
- For a hang-on unit, seat the rim hook, fit the airline, and run the air stone until you see a steady trickle.
- Net the adult gently, cover the box while it settles, and keep the tank light low for the first hour.
- Feed a pinch, siphon crumbs after ten minutes, and watch that the fish can turn and rise to the surface.
- Remove the box as soon as fry are eating or the adult is calm enough to return, then dry the parts so mesh does not sour.
If the fish gasps or hangs in a corner, open more vents or move it back to the main volume. A box is a fence, not a hospital filter. Keep a lid or cover on a hang-on cell so a startled guppy cannot jump onto the floor.
Frequently Asked Questions
Do You Need a Separate Heater in the Box?
Acrylic and mesh boxes sit in the tank water, so the main heater covers them if flow is open. The Fluval Hang-On Breeding Box needs an air pump to pull that warm water through. It does not include a heater.
Can Two Adults Share One Acrylic Box?
Only if each room is large enough to turn. The 17.3 cm Uniclife and Pawfly boxes make three rooms. A full-grown pair in one 8 cm cell will fight or stall.
Will Fry Escape Through Mesh?
The Jesiah and MDLSDE nets state 1 mm holes. Newborn livebearers usually stay in. Eggs and tiny shrimp larvae may not. Check the mesh against the species you spawn.
Can You Leave a Box in Place All Month?
No. Waste collects, cups slip, and the fish cannot swim a normal path. Take it out when the job is done and store it dry.
Is a Breeding Box a Substitute for a Quarantine Tank?
It can hold a fish for a short look. It cannot give you separate water chemistry. A true quarantine needs its own cycled volume, not a cup on the display glass.
Last reviewed: 5 October 2026.
Fish Breeding Boxes to Compare
Fluval Hang-On Breeding Box
A 16 cm, 1.1 litre hang-on cell with a V-separator so fry drop away from the adult, using an air pump you supply to move water from the main tank.
Uniclife Acrylic Breeding Box
A floating acrylic hatchery with an inner 17.3 x 10 x 10 cm chamber, two dividers that make three rooms, side vents and two suction cups.
Pawfly Large Fish Breeding Box
A larger 20.3 x 11.7 x 13.2 cm acrylic floater with two dividers, six suction cups and a double-sided cup so the box sits off the glass and leaves a gap.
Pawfly Acrylic Breeding Box
The 17.3 x 10 x 10 cm Pawfly acrylic floater with two dividers and two cups, the same inner size as the Uniclife box in a different moulding.
QWORK Aquarium Fish Breeding Box
A compact clear polypropylene box at 127 x 65 x 70 mm with side vents, removable separators and suction cups, sized for shrimp or fry rather than a large adult.
Plomveriq Acrylic Breeder Box
A self-floating acrylic cell about 8 x 7 x 11 cm with an isolation plate and suction cups, for a single small adult or a handful of fry.
Jesiah Fish Breeding Box
A 16 x 14.5 x 15.5 cm nylon-mesh crate with a plastic frame, two dividers, 1 mm holes and four suction cups, stated for 35 to 150 litre tanks.
Tabanzhe Fine Mesh Hatchery
A fine-mesh hanging net with six suction cups and a clear view through the weave, for fry or a resting fish when you want more flow than a solid acrylic wall.
IRYNA Fry Hatchery Net
A 26 x 15 x 15 cm polyester-mesh hanging crate with six suction cups, the largest net in this set, for a bigger adult or a larger fry group.
MDLSDE Fish Breeder Net
A 16 x 14 x 15 cm nylon-mesh box with 1 mm holes, two dividers and four cups, stated for 35 to 150 litre tanks as a three-chamber nursery.