Caring for Baby Fish

Planning Ahead Before the Fry Arrive

Many aquarium fish eat their own eggs or fry, and tankmates usually will too. A successful spawning can leave you with a tank of tiny, vulnerable young and little time to decide what to do. Preparing before breeding happens gives far more of them a chance.

Start by reading up on your species. Some fish provide parental care, including many cichlids, some catfish and various labyrinth fish. Whether to leave a parent with the brood depends on the species, the parents' behaviour and the aquarium setup. A good local aquatic shop or fishkeeping club can usually offer species-specific advice.

With livebearers such as guppies, platies, mollies and swordtails, a female's belly often swells before she gives birth. This is not a reliable countdown, and swelling can have other causes. If males and females share a tank, assume fry could appear at any time. Females can also store sperm and produce further broods after being separated from males, so separation will not stop births straight away. If you want fewer fry, consider stocking choices such as single-sex groups.

Live foods take time to produce, so start infusoria cultures or organise brine shrimp hatching before the fry need feeding.

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Where to Keep Baby Fish

Fry can survive in the main aquarium, but survival is unpredictable, particularly where adult fish can reach them. Dense planting, floating plants and decorations with small gaps all help. Fine-leaved plants such as java moss are especially useful, as fry can hide among them and pick at tiny organisms on the leaves. Fit a sponge guard to any accessible filter intake.

A separate rearing tank gives more control, provided it is mature and properly maintained. It need not be elaborate, but its size must suit the species and number of fry, with room to move them into larger accommodation as they grow. The basics are:

• A sponge filter, with an air pump and airline, which will not suck fry in
• A heater set to suit the species, plus a thermometer
• Live or artificial plants for cover
• A lid, as some young fish jump

Set this tank up before breeding. Waiting alone does not cycle a tank: a fishless cycle needs an ammonia source and regular testing, and usually takes several weeks. Mature filter media from a healthy aquarium can help establish filtration, but do not remove so much that the original tank's filtration suffers. Old aquarium water alone will not cycle anything. A spare sponge filter kept running in your main tank can shorten preparation considerably, although its capacity is limited. Avoid moving fish or equipment from any tank showing signs of disease.

If fry arrive unexpectedly and you have no mature tank, do not move them into an uncycled one. Give them cover in the main aquarium or use a nursery box for the short term.

Breeding boxes and nets that hang inside the main tank are cheap and easy, but they are temporary holding arrangements. Sharing the tank's water does not guarantee good exchange inside the box, so check regularly for crowding, poor flow, waste and uneaten food. Avoid routinely confining a pregnant female in a small breeding trap, as this can cause stress.

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Feeding and Water Quality

Many newly hatched egg-layers live on their yolk sac at first. Feeding usually begins once it is absorbed and the fry are free-swimming, though this varies by species. Food added earlier simply pollutes the water. After that, several tiny feeds a day usually work better than one or two larger ones.

Match first foods to fry size:

• Larger fry, including most livebearers: suitably sized powdered foods, finely crushed flake and newly hatched brine shrimp
• Very small fry, common among egg-layers: microscopic live foods such as infusoria or suitable rotifers, depending on species, as microworms and brine shrimp can be too large at first

Commercial liquid fry foods vary in suitability and nutritional value, and overuse fouls the water.

Young fish are sensitive to ammonia and nitrite, and frequent feeding builds waste quickly. Use tests for ammonia, nitrite and nitrate, as many basic strips omit ammonia. Test regularly and adjust partial water changes to the stocking and feeding level; some fry tanks need daily changes. Aim for no detectable ammonia or nitrite, act promptly if either appears, and monitor nitrate. Use dechlorinated replacement water matched closely to the tank's temperature and chemistry. Remove leftovers with a small siphon or pipette, taking care not to catch any fry.

Growing On and Finding New Homes

Fry grow at different rates, and stronger ones may outcompete or bully smaller siblings. Watch feeding times so all get a share, and split them up if size differences become large. Move them gradually on to larger foods.

Being too big for adults to swallow is only one condition for returning juveniles to the main aquarium. They can still be injured, harassed or beaten to food. Check tankmates' temperament, water requirements and whether the tank has room, and acclimatise the young fish properly.

Plan homes before numbers get out of hand. Ask shops in advance, as many cannot accept home-bred fish, and never leave fish at a shop without agreement. Fishkeeping clubs often hold auctions or swaps.

Never release fish into wild or outdoor waters where they could escape or harm native wildlife. This is damaging and can be unlawful, and most tropical species cannot cope with UK outdoor conditions anyway. If you intend to breed or sell fish as a business, check animal-activity licensing requirements with your local council, as rules differ across the UK. The RSPCA and the Ornamental Aquatic Trade Association both publish welfare guidance worth reading.

Keep testing the water as the fry grow, and know where each batch will go before the next one arrives.

The Next Step

Now that you have read through the advice above, you might want to put it into practice. Our Aquarium Stocking Calculator lets you calculates safe fish stocking levels based on tank size, filtration, and species bioload. Try it now →