My fish had babies out of nowhere — what do I do?

You almost certainly have a livebearer (guppy, platy, molly or swordtail). The fry are at risk of being eaten, so either provide dense cover (java moss, floating plants, breeding grass) for them to hide in, or move the fry (or the pregnant mother) to a separate container. Feed them tiny food several times a day, keep the water clean — or, if you don't want to raise them, let the community tank naturally control the numbers.

How can I tell if my livebearer is pregnant?

Look for a dark 'gravid spot' near the back of the belly that enlarges and darkens as she nears giving birth, and a boxy, swollen belly. Livebearer pregnancies last roughly 21–30 days, and a female can deliver batch after batch because she stores sperm from earlier matings.

Do I need a separate breeding or fry tank?

For livebearers it's optional — dense planting lets some fry survive in a community tank, though a breeding box or nursery tank raises far more. For egg-laying species it's usually essential, because parents commonly eat the eggs and fry, and you need to control conditions and feeding.

What do baby fish eat?

Tiny food, often several times a day. Newly-born fry take infusoria, liquid fry food, or very finely crushed flake for the first days; livebearer fry are big enough to eat baby brine shrimp almost immediately, while tiny egg-layer fry work up through infusoria → micro worms → baby brine shrimp. Clean water and small, frequent feeds matter most.

How do I stop my fish breeding so much?

Keep an all-male (or single-sex) group, don't overstock, and in a community tank let the other fish naturally eat the surplus fry so numbers self-limit. Don't keep adding more livebearers — a mixed group grows fast on its own. Remember the females already carry stored sperm, so removing the male won't stop existing pregnancies.

Should I breed my fish on purpose?

Only if you can house or rehome the young responsibly, and ideally with a plan for healthy stock. Breeding is rewarding, but fry add up fast, and careless breeding (heavy inbreeding, random hybridising) produces weaker fish — an issue already seen in mass-bred guppies and neon tetras. Breed with intention, not by accident.

Breeding fish and raising fry: from 'help, my guppy had babies' to doing it on purpose

The 30-second version. There are two kinds of breeders. Livebearers (guppies, platies, mollies, swordtails) breed constantly and give birth to live fry — often as a surprise, because females store sperm and keep having batches for months. Egg-layers usually need deliberate conditioning and a dedicated tank, and many eat their own eggs. When fry appear, protect them with cover or a separate container and feed tiny food often. Only breed on purpose if you can rehome the young.

For most people, the breeding chapter of fishkeeping opens with a jolt: you look in the tank and there are tiny fish in there. Congratulations — you (well, your fish) have discovered the easiest breeding project in the hobby. Here’s how it all works, and what to actually do.

Two kinds of breeders

Freshwater fish split neatly into two reproductive camps, and they need completely different handling:

Livebearers — guppies, platies, mollies, swordtails (and Endlers). The female carries the young and gives birth to free-swimming live fry. They breed readily, often without you doing anything at all.
Egg-layers — most other fish (tetras, barbs, corydoras, cichlids, bettas…). They lay eggs, which may be scattered, deposited on surfaces, or (in some cichlids) guarded by parents. Getting these to breed usually takes intention.

The livebearer surprise

If babies appeared unannounced, you have livebearers — and one fact explains it: a female stores sperm from a single mating and keeps producing batch after batch of fry for months, even with no male in sight. So “just remove the male” doesn’t stop the pregnancies already in the pipeline.

Signs a female is expecting: a dark, enlarging “gravid spot” near the back of the belly, and a boxy, swollen shape. Pregnancy runs roughly 21–30 days, and each brood can be a handful to dozens of fry.

The catch: the adults will happily eat the fry. In nature most young are lost; in your tank, whether any survive is up to you.

When fry appear: protect and feed

Two decisions — protect them, and feed them:

Protect: give fry dense cover to hide in (java moss, hornwort, floating plants, or breeding grass), or move the fry — or the pregnant mother — into a breeding box or a small nursery tank. Separating them prevents predation and lets them grow faster with less competition. Return fry to the main tank once they’re around half an inch, too big to be a snack.
Feed: small food, several times a day. Livebearer fry are large enough to take baby brine shrimp almost at once, plus finely crushed flake and infusoria. Keep the water pristine with gentle (sponge) filtration and small, frequent water changes, and keep them warm (24–27 °C / 75–81 °F).

Don’t want to raise them? That’s a valid choice — in a planted community tank, the other fish eat most of the surplus and the population self-limits.

Breeding egg-layers on purpose

Egg-layers rarely “just happen” — you set the stage:

Condition the breeders for 1–2 weeks with protein-rich live/frozen foods, stable water, and triggers like water changes and small temperature shifts.
Use a dedicated breeding tank. Many egg-scatterers don’t guard their eggs and will eat them, so you separate the parents after spawning (or use a mesh/marble base the eggs fall through out of reach).
Feed the tiny fry a ladder: infusoria or liquid fry food for the first few days → micro worms → baby brine shrimp once they’re big enough (usually a couple of weeks). Once fry take baby brine shrimp, raising them gets much easier.

Different species have their own quirks (a betta’s bubble nest, a corydoras’ T-position spawn, a cichlid’s parental care), so read up on your specific fish before you start.

The honest welfare part

Breeding is genuinely rewarding — but do it with intention:

Have a plan for the fry. They multiply fast; know whether you can house them, or whether a local shop or fellow keeper will take them, before you start.
Breed for health, not just quantity. Heavy inbreeding and careless hybridising produce weaker, disease-prone fish — a problem already visible in some mass-bred guppies and neon tetras. Choose good stock and avoid mixing closely-related species.
Don’t dump surplus fish. Never release aquarium fish into the wild.

Where to go next

Fry need pristine water above all — see water changes and feeding.
Thinking about which fish to keep (and breed)? Start with the species guides — guppies are the classic first breeding project.

Sources & further reading

Aquarium Co-Op — How to Raise Baby Fish Fry — protecting fry, fry food, and raising survival rates.
Aquarium Co-Op — How to Breed, Hatch, and Raise Egg-Scattering Fish — conditioning, dedicated breeding setups, and the egg-eating problem.
SeriouslyFish — Poecilia reticulata (guppy) reproduction — livebearer breeding behaviour and the several-females-per-male guidance.