Fish Breeder Box vs Breeder Net: Fry Saver Sizing and What the Slot Width Means

A fish breeder box is a small container that holds fry apart from the rest of the tank. A breeder net does the same job with soft mesh instead of rigid walls, so a box keeps its shape and a net sags with the water around it. That is the whole difference in one line.

This guide covers when to pick a box over a net, what a slot width number really tells you, how our two mounts work, and how long keepers actually leave fry in one. One thing first, so you land on the right page. A breeder box keeps small fish inside a space. An intake guard keeps them out of a filter. Those are two different problems and two different parts. If your fry are getting pulled into a filter, our shrimp guard guides cover that instead.

What is a breeder box, and what is it for?

A breeder box is a slotted container that sits in or on your tank and holds fry, a pregnant fish, or an injured fish away from the rest of your stock. The slots let water pass through the walls, so the box shares the tank's water instead of holding its own separate pool.

Keepers use one for a few reasons. Fry are small enough to be eaten by their own tank mates. A fish healing from a nip or a fin tear needs a break from the crowd. A livebearer close to dropping fry needs somewhere the fry will not scatter into the gravel.

It is not a filter part. An intake guard is a slotted cage that clips over a filter intake to keep small animals out of the impeller. A breeder box is a container that keeps small animals inside it. Same idea, opposite direction. Our fish guard and shrimp guard guides cover the intake side, so we do not repeat that here.

Breeder box or breeder net: which one do you need?

Pick a rigid box when you want the walls to stay put and the gap size to stay the same everywhere. Pick a net when you want something cheap that folds flat when you are done. A net is soft, so its shape changes with the water and with whatever leans on it.

There is a third option people forget. A small grow out tank of its own gives fry far more room than any box will. It also means another heater, another filter, and another set of water changes.

What does the slot width on a fry saver mean?

The slot width is the width of the opening cut into the wall, measured in millimetres. Our boxes come two ways. The slots measure 1.5 mm on one version and 3 mm on the other. Every slot is 8 mm tall. The left, right and front walls carry them, the floor carries a second set, and the back wall stays solid because it faces the glass.

Here is the part almost every listing skips. We picked 1.5 mm and 3 mm as design choices. We have not tested either width against live fry, and no source we could find publishes a fry exclusion gap for any species. So we tell you the number our part measures and we stop there. What we will not tell you is that fry cannot get through it, because we do not know that and neither does anyone selling you a box without a number on it.

Our shrimp guard uses the same 1.5 mm slot. That number was chosen for the same plain reason, a small opening. It has not been tested for holding fry either, and one part passing a fit test does not prove anything about the other.

So how do you judge it yourself? Put a ruler with millimetre marks up against the wall of the box and look at the real gap, rather than picturing the number. Then watch the box for the first day after you set it up, and count what is in there. You know your fish and the size they were born at. We do not.

Hang on, in tank, or floating: which mount fits your setup?

We make two mounts. A rim hang version clamps over the tank wall. A suction version hangs on cups inside the tank. We do not make a floating one, and the reason is worth stating.

The rim hang version has a channel that drops over the wall, with an outer skirt on the far side. The channel opens to 10.8 mm. An M5 nylon thumb screw threads through the skirt and snugs the channel down onto walls from about 2 mm thick upward, so it is an adjustable range and not a fit for one tank. Measure your own wall first. The screw is customer supplied and is not included with the part.

The suction version has two keyholes through the solid back wall. The round opening measures 11 mm and the narrow stem slot measures 5 mm. The cup head pushes through the round part, then the box slides down and traps the head behind the wall. Those two openings are our own design choice. We did not source a cup manufacturer's spec, and hobby cups vary a lot by brand, so measure your cups before you order and do not assume. Cups are customer supplied and are not included.

A floating box needs a sealed air pocket to stay up. A printed part with a sealed air pocket has to hold air under water for weeks, and we have not tested one that does. Floating is also a claim about how the part behaves, and we do not make claims we have not tested. So we did not make one rather than sell one we could not stand behind.

Do breeder boxes need their own water flow or an air pump?

Ours does not use an air pump, and no pump or air line is included. Water moves through the slots in the three walls and the floor. That is the whole mechanism.

Some other boxes do use an air line. The air rises inside a small tube and lifts tank water up into the box with it. That design pushes water through on purpose instead of letting it pass through openings.

We do not publish a water exchange figure for our box, because we have not measured one and we are not going to print a number we did not test. What we would do in your shoes is watch it. Check the box the first day and the first week. If the water inside looks or smells different from the tank around it, act on that, not on anything a product page told you.

How long should fry stay in a breeder box?

There is no single number, and any page that gives you one is guessing. How long depends on the fish, how fast they grow, and what else lives in your tank. The working rule most keepers use is simple: fry come out when they are too big to fit in a tank mate's mouth.

That is a judgement call, not a date on a calendar. A few things make the call easier.

• Compare the fry against the mouth of the largest fish in the tank, not against the tank's average fish.
• Watch how crowded the box looks as they grow. A box that was roomy in week one may not be in week three.
• Plan where they go next before you need it. A grow out tank, a divided section, or a rehome all take setup time.
• If you are unsure, ask in a keeper forum with your species and your tank list. Real answers beat a generic number.

Why fry get stressed in a small box, and what to do about it

A box is a small space, and small spaces get dirty fast. Uneaten food and waste build up in a low volume of water much quicker than they do in the tank around it. Add little room to swim and nowhere to hide, and you get the stressed fish keepers write about in forum threads.

This is the honest reason a breeder box is a short term tool and not a home. It buys you time while fry grow. It is not a smaller tank.

A few things help. Pick the standard size over the nano if you have the rim space for it. Keep the count in the box lower than you think you need to. Feed small amounts and siphon what does not get eaten. And move them out as soon as the size call says you can.

How to clean a breeder box without losing fry

Clean it in place with a siphon rather than lifting it out. Most fry get lost during the lift, not during the clean. A short length of airline tubing gives you a slow, thin pull that is much easier to control than a gravel vacuum.

1. Rinse the box under tap water before its first use, the same as any new part that goes in your tank.
2. Turn off anything that makes the surface choppy while you work, so you can see into the box.
3. Start a siphon with a short piece of airline tubing into a light coloured cup or bowl.
4. Draw waste off the floor slowly. Keep the tube tip moving so it does not park next to a fry.
5. Hold the cup up to the light and check it before you pour anything out. Fry are easy to miss against a dark bucket.
6. Pour anything you find back into the box, then top the tank back up if the level dropped.

Frequently asked questions