There is a point where saving a few fry in a jungle of floating plants stops being enough. Maybe you have decided to raise a proper batch of guppies, maybe you want to breed a particular strain on purpose, or maybe the broods are simply too big to leave to chance. At that point you need a dedicated space — and the good news is that a grow-out tank is one of the cheapest, simplest tanks you will ever set up, because its whole job is to be stable and safe rather than beautiful.
This is the companion to what to do when your fish have babies. That guide covers the biology and the decision of whether to raise them; this one is the practical build — the filter, the heater, the water, and how to keep a nursery from crashing in a small flat.
Why a separate tank beats a breeding box
A breeding box hanging in your main tank is fine for a handful of fry for a week or two. For a whole brood raised to sellable size, it falls apart: too small, fouls fast, and cramped water is exactly what fry cannot tolerate. A dedicated tank solves every one of those problems at once.
| Option | Fry capacity | Water stability | Best for |
|---|---|---|---|
| Floating plants in main tank | A few survivors | Excellent (big volume) | Casual keepers who want some, not all |
| Breeding box in main tank | Small batch, short term | Poor (tiny volume, fouls fast) | A week or two of protection, nothing more |
| Dedicated grow-out tank | A whole brood to adult | Good, if you manage it | Raising most of a brood; deliberate breeding |
The other quiet advantage: a grow-out tank is not single-purpose. Between broods it becomes a quarantine tank for new arrivals or a hospital tank for a sick fish. Many experienced keepers keep a small bare tank permanently on standby for exactly this reason. It earns its shelf space.
The principle: simple, stable, safe — not a showpiece
Forget aquascaping here. A grow-out tank has three jobs: keep the water clean, keep the temperature steady, and let you see and feed the fry easily. Everything about the setup follows from those three, and almost every beginner mistake comes from trying to make it pretty instead.
Size and where it goes
You do not need much. Something in the 20–60 litre range raises a typical livebearer brood comfortably, and even a 10–20 litre nano tank works for a small batch if you are diligent with water changes. Bigger is genuinely easier, because more water is more stable — but in a small flat, a modest tank on a sturdy shelf is the realistic choice.
Wherever it goes, the same placement rules as any tank apply: out of direct sun, away from the worst of the air-conditioning draught, and on something that can take the weight of water. If it is going on furniture, glance at the load first — the arithmetic is in the stand and cabinet guide for full tanks, and a grow-out tank is lighter but not weightless.
The sponge filter: the heart of the whole thing
If you take one thing from this guide, take this: a fry tank runs on a sponge filter, not a normal filter. This is not a budget compromise — it is the correct tool, and here is why.
- It cannot suck up fry. A powered filter intake will hoover newborn fish straight into the impeller. A sponge filter draws water gently across a large sponge surface — fry are never at risk, and in fact they graze on the sponge.
- It is a biological filter and a food source at once. The sponge grows a film of microorganisms (infusoria and biofilm) that newborn fry pick at constantly between feeds. You are filtering the water and feeding the babies with the same object.
- It is driven by air, not a motor. A sponge filter runs off an air pump, which also keeps the water well oxygenated — useful in warm UAE tanks where oxygen is already at a premium.
- It is almost impossible to get wrong. No settings, no flow to tune, nothing to break. You squeeze it out in old tank water during a water change and drop it back.
Cycle it before the fry arrive
A brand-new sponge filter is biologically empty, and an uncycled tank will poison fry as surely as adults. The fix is easy and it is the single most important preparation step: seed the sponge from an established tank. Run the new sponge filter in your main tank for a couple of weeks before you need it, or squeeze some gunk from your main filter into the fry tank, or move a handful of established media across. That transplants the bacteria colony and gives you a cycled nursery from day one. If none of that is possible, you must cycle the tank properly first — the full method is in how to cycle a new aquarium, and the wider logic of why filters matter is in filtration basics. Never put fry into an uncycled tank and hope.
Heater and temperature
Warmth drives growth. Fry raised a touch warmer grow faster and more evenly, and stable temperature matters more than the exact number. A small adjustable heater keeps a nursery steady — but the UAE adds a twist most breeding guides ignore.
For much of the year, a small tank in an air-conditioned flat may need little or no heating, and in summer the real risk is the opposite: a small volume overheating during the day when the AC is off. A grow-out tank's low water volume swings temperature faster than a big display tank, so it is more vulnerable to a hot afternoon, not less. Keep an eye on it, and if it is creeping into the low thirties, the tactics in keeping an aquarium cool in summer apply doubly to a nursery.
| Season / situation | Temperature risk | What to do |
|---|---|---|
| Winter, AC off | Stable, may need gentle heating | Small heater set modestly; check it holds steady |
| Summer, AC running | Can run cool from the draught | Keep away from vents; heater evens it out |
| Summer, AC off in the day | Small volume overheats fast | Watch closely; a cooling fan or reduced lighting helps |
Bare bottom or planted?
Both work, and the choice is a genuine trade-off rather than a right answer.
| Bare bottom | Lightly planted | |
|---|---|---|
| Cleaning | Very easy — siphon waste and uneaten food off the glass in seconds | Harder — debris settles among plants |
| Feeding visibility | Excellent — you see exactly how much fry are eating | Food gets lost in the foliage |
| Fry cover and security | Low — fry feel exposed | High — fry feel safe and colour up better |
| Extra grazing food | Only the sponge | Sponge plus plant biofilm |
A common middle path is a bare bottom for easy cleaning plus a few handfuls of floating plants or a clump of moss for cover. That gives you feeding visibility and waste control without leaving the fry feeling exposed. Cheap, hardy plants suit this perfectly — the hard-water plant list covers species that grow in UAE tap water with no fuss, and floating plants in particular double as the fry cover discussed in the companion guide.
Keeping the nursery stable: the water is everything
This is where grow-out tanks are won or lost. You are combining the two things that most stress a tank: a small volume and heavy feeding. Fry need feeding several times a day, and all that food becomes waste in not much water. Left alone, ammonia climbs and the fry — far more sensitive than adults — suffer first.
The answer is frequent, small, careful water changes.
| Rule | Why |
|---|---|
| Change water little and often — small amounts every day or two | Keeps waste low without shocking the fry with a big swing |
| Match the new water's temperature to the tank | Fry are highly sensitive to temperature swings; a cold refill can chill them |
| Treat/age the water first, as always | The same tap-water rules as any tank — see how to do a water change |
| Siphon slowly with fine tubing or over a net | So you do not vacuum up the fry themselves |
| Never overfeed — uneaten food is the number-one killer | Feed tiny amounts the fry clear in a minute; remove what they miss |
If you keep a test kit on hand, a grow-out tank is where it earns its keep — an ammonia reading creeping above zero is your early warning to change more water and feed less. In a nursery, that number is the difference between a thriving batch and a sudden overnight loss.
Feeding in a grow-out tank
The feeding regime is covered in the companion guide, but the grow-out-specific point is rhythm: little and often, and always matched to your water changes. More feeds mean faster growth but more waste, so the two must move together. If you step feeding up, step water changes up too. Crushed flake, powdered fry food and — if you are keen — live baby brine shrimp are the staples; the general principles are in what to feed aquarium fish, just finer and more frequent.
Sorting fry by size as they grow
Here is something beginners rarely anticipate: fry from the same brood grow at very different rates, and in a grow-out tank the biggest quickly become a threat to the smallest. A large fry will eat a much smaller sibling without hesitation — same instinct as the adults. If your batch is spreading out in size, grade them.
- Move the largest, fastest-growing fry out to the main tank once they are too big to be eaten there — this frees space and removes the bullies.
- If you are raising several broods, keep them in separate size groups rather than one mixed tank.
- Uneven growth often signals uneven feeding or crowding; a little more space and more frequent small feeds evens it out.
When to move them on
Grow the fry until they are past mouth-size for your adults — usually two to three months, when they are around 2–3 cm and showing colour. At that point they can join the main community, be given away, or be offered to a local shop. Do not let a grow-out tank quietly become a second overstocked display tank; the whole point is that fish move through it. And the disposal rule from the companion guide is worth repeating once: rehome or return surplus fish, but never release them into any UAE waterway.
If you are breeding on purpose
Everything above assumes you are raising an accidental brood. If you actually want to breed — to fix a colour strain of guppy, say — the same tank is your tool, with a little more intent:
- Choose your parents. Select the healthiest, best-coloured, best-shaped adults rather than breeding from whatever you happen to have. A trio of one male to two or three females is a standard starting group.
- Condition them first. A week or two of excellent food and clean water before breeding produces bigger, healthier broods.
- Keep strains separate. If you want predictable colours, do not let different strains interbreed — which in practice means separate tanks or single-strain groups.
- Keep records if you are serious. Even a note on which parents produced which batch helps you improve over generations.
Deliberate breeding is a genuinely rewarding rabbit hole, and livebearers are the ideal starting point because they make it so easy. If you catch the bug, breeding invertebrates like cherry shrimp is the natural next step — and a colony of shrimp is raised on very similar principles: a stable, gently-filtered, well-planted tank.
The kit checklist
| Item | Needed? | Notes |
|---|---|---|
| Small tank (20–60 L ideal) | Yes | Bigger is more stable; nano works with diligence |
| Sponge filter | Yes — non-negotiable | The one piece you must not substitute with a normal filter |
| Air pump + airline | Yes | Drives the sponge filter and oxygenates |
| Small heater | Usually | For stable temperature; watch for summer overheating instead |
| Floating plants / moss | Recommended | Cover, security and grazing biofilm |
| Fine-tube siphon | Yes | For gentle water changes that do not suck up fry |
| Fry food (crushed flake at minimum) | Yes | Powdered food or baby brine shrimp optional |
| Test kit | Strongly recommended | Your early warning against ammonia in a small, heavily-fed tank |
| Established filter media to seed | Yes | The shortcut to an instantly cycled tank |
Frequently asked questions
Do I really need a separate tank, or will a breeding box do?
For a few fry over a week or two, a box is fine. For raising a whole brood to sellable size, a box fouls too fast and cramps the fry. A small dedicated tank is far more forgiving and doubles as a quarantine tank the rest of the time.
Why can't I just use a normal filter turned down low?
Even on a low setting, a powered intake sucks in newborn fry. A sponge filter physically cannot, and it doubles as a grazing surface. It is the right tool, not a compromise.
How do I cycle a fry tank quickly?
Seed it. Run the sponge filter in an established tank for a couple of weeks beforehand, or squeeze mature filter gunk into it, or move over some established media. That transfers the bacteria you need. Without seeding, cycle it fully before any fry go in.
My fry are different sizes and some are disappearing. Why?
The bigger fry are eating the smaller ones — normal livebearer behaviour. Move the largest out to the main tank once they are big enough, or separate the batch into size groups.
How often should I change the water in a grow-out tank?
Little and often — small changes every day or two, always temperature-matched. A small tank with heavy feeding needs more frequent attention than a display tank, not less.
Can I keep the grow-out tank running empty between broods?
Yes, and many keepers do — a running sponge filter stays cycled and ready. It makes an ideal permanent quarantine or hospital tank, so it is rarely truly empty for long.
Is breeding livebearers a good first breeding project?
It is the best one. They breed readily, the fry are large and easy to feed, and the local hard water suits them. If you want to learn to breed fish, start here before anything that needs special conditions.
The short version
- A dedicated grow-out tank beats a breeding box for raising a whole brood — and doubles as a quarantine tank.
- Keep it simple: stable water, steady warmth, easy feeding. Not a showpiece.
- A sponge filter is the heart of it — it cannot eat fry, it feeds them biofilm, and it runs off an air pump. Do not substitute a normal filter.
- Seed the filter from an established tank so the nursery is cycled before fry arrive.
- Watch temperature: a small volume overheats fast on a hot UAE afternoon.
- Bare bottom is easiest to clean; add floating plants or moss for cover.
- Small, frequent, temperature-matched water changes are what keep a heavily-fed nursery alive.
- Grade fry by size — big ones eat small ones — and move them on at 2–3 cm.
- If breeding on purpose, choose and condition your parents and keep strains separate.
If you arrived here because your tank produced fry unexpectedly, start with the companion guide: what to do when your fish have babies. And once the nursery is running smoothly, keeping the parent tank in good order is just ordinary aquarium maintenance — now with more mouths to feed.