Fish Breeding

People usually end up searching fish breeding advice for one of two reasons: a pair has started spawning in the community tank, or you’ve decided to breed a species properly and you don’t want to lose eggs and fry to bad water, hungry tankmates, or avoidable disease.

In an aquarium, breeding success is less about luck than control. Water quality, gentle filtration, and a plan for what happens to surplus young fish matter as much as the actual spawn. Done well, captive breeding can take pressure off wild collection and reduce the urge to “top up” stocks from the wild.

Before you breed: ethics, planning, and biosecurity

Breeding fish produces living animals quickly. The practical question isn’t “can they spawn?” but “can you raise healthy juveniles and place them responsibly?” Planning early prevents overcrowding, poor growth, and needless losses.

• Know your outlet for juveniles before you set a pair up. Even hardy livebearers can overwhelm a tank in weeks.
• Never release aquarium fish, plants, or water into creeks, rivers, dams, or ponds. Escapes and releases can spread pests and disease and are hard to undo once established.¹
• Don’t assume imports will “soon be banned”. Australia already tightly regulates live ornamental fish imports through permits, approved species lists, and quarantine/approved arrangements, and conditions can change over time.^{2, 3}

Choosing parent stock: quality, health, and genetics

Strong fry begin with strong parents. Select breeding fish that are robust, well-grown, and free of obvious deformities (twisted spines, damaged mouths, persistent fin deformities). Avoid breeding from “the runt you felt sorry for” if the issue looks structural rather than temporary.

Genetics matters in small aquarium populations. Random inbreeding can fix unwanted traits and reduce vigour over time, while planned line breeding can be useful when you have a clear goal and keep careful records. If you can, start with unrelated fish from healthy lines and rotate in new blood occasionally.

The breeding tank: making conditions predictable

A community tank is designed for compromise. A breeding tank is designed for outcomes: stable water chemistry, low stress, and controlled access to eggs and fry.

• Match water chemistry to the species (temperature, pH, hardness, salinity where relevant). Many eggs fail simply because the water is “fine for adults” but wrong for embryos.
• Prioritise biological filtration and oxygenation. Fish waste becomes ammonia, which is highly toxic; an established biofilter converts ammonia to nitrite and then nitrate.⁴
• Keep it mature and steady. A newly set up tank is often the most dangerous place for fry because the biofilter is still developing.⁴

Spawning triggers (when nothing happens)

Some species respond to seasonal cues. In practice, aquarists often simulate these changes carefully:

• frequent partial water changes (the “fresh rain” effect)
• an upswing in high-quality foods (often live or frozen)
• small, deliberate temperature adjustments within the species’ safe range
• for some species, longer day length and brighter lighting

Once the fish are settled, avoid constant “tweaking”. Stability is a breeding trigger in its own right.

Egg survival: why most spawns vanish in a community tank

Most fish don’t guard eggs and will eat them opportunistically, as will their tankmates. In an aquarium, eggs are concentrated, easy to find, and rarely survive without protection.

Some groups (notably many cichlids) may guard eggs and fry, but their defensive behaviour can make them unsafe to keep with other fish during breeding. Separate quarters are often the calm option.

Types of breeding setups (and what they’re for)

Egg scatterers (barbs, danios, many tetras)

• Marble layer: a thin layer of marbles lets eggs fall out of reach. Keep the water level lower if appropriate for the species to reduce the time eggs are exposed while sinking.
• Mesh/egg grate: parents spawn above a fine mesh; eggs drop through to safety.
• Plant thicket: fine-leaved plants (or spawning media) trap adhesive eggs and provide cover until you can remove adults.

Egg depositors (many rainbowfish, some killifish)

Spawning mops (acrylic yarn mops or similar) provide a predictable place for eggs. Eggs can be collected and moved to a hatching container if you need maximum control.

Livebearers (guppies, platies, mollies)

Breeding traps are widely sold, but they can stress the female and are easy to misuse. If you use one, treat it as a short-term tool, not housing.

• Use a trap only when the female is close to dropping fry.
• Return the female to a roomy tank soon after birth so she can recover.
• Where possible, a separate, planted nursery tank is gentler than a cramped trap.

Rearing fry: water quality first, food second

Fry are small, sensitive, and easily poisoned by poor water. They’re also easy to injure with strong filtration and rough handling.

Water quality and filtration

Biological filtration is the backbone. Ammonia and nitrite must stay at zero; nitrate should be kept low with regular partial water changes.⁴

For very small fry, use gentle filtration and protect intakes. Air-driven sponge filters are a common choice because they provide biological filtration with low suction and steady aeration.

Use “chemical media” with care. Products like zeolite can temporarily bind ammonia, but they don’t replace a mature biofilter, and they can mask a problem you still need to fix at the source.

Handling fry without damage

Avoid moving fry unless you must. When transfer is unavoidable, move slowly, keep temperature consistent, and use soft tools. A small cup or a turkey baster-style tool is often safer than a net for tiny fry.

Feeding fry: matching food size to mouth size

After hatching, many larvae live briefly off their yolk sac, then become free-swimming and need food small enough to swallow, offered often enough that they don’t starve between meals.

First foods

• Infusoria/protozoans: useful for the tiniest fry. Cultures are easy to crash, so start early and run more than one culture at a time.
• Rotifers: often used in marine larviculture and for very small larvae where brine shrimp are too large.⁵

Brine shrimp (Artemia) nauplii

Newly hatched brine shrimp are one of the most common live foods in aquaculture and home breeding. Under typical hatchery conditions, cysts hatch in roughly a day, and nauplii are then harvested and fed.⁵

Two useful cautions:

• Size and suitability: brine shrimp nauplii can be too large for many first-feeding marine larvae; they’re often introduced after an earlier live-feed phase (such as rotifers).⁵
• Nutrition (especially marine species): Artemia may need enrichment for demanding marine larvae because their fatty acid profile can be inadequate without it.⁵

Microworms and other small live foods

Microworm cultures are popular because they’re cheap and productive. Maintain clean cultures, restart regularly, and avoid feeding sour or contaminated cultures to fry.

As fry grow, transition to larger items (larger live foods, finely crushed flakes or pellets) in step with mouth size and swimming ability.

Culling, surplus fish, and humane decisions

Selective rearing is part of responsible breeding. If you keep every fry regardless of health or form, you can quickly end up with overcrowding, stunted growth, and poor welfare. Plan to raise fewer juveniles well, rather than many poorly.

If you ever face the need to humanely end suffering, follow recognised humane killing principles: the method should produce rapid loss of consciousness and minimise pain and distress, and it must be appropriate to the species and life stage.⁶

Marine breeding: possible, but still hard at home

Marine ornamentals are being bred in increasing numbers worldwide, but raising larvae remains much more demanding than most freshwater species. The early feeding chain (often algae → rotifers → Artemia/enriched feeds → weaning) is where many home attempts fail.⁵

References

1. Queensland Government – Prevent the spread of invasive freshwater animals (including aquarium fish)
2. Australian Government Department of Agriculture, Fisheries and Forestry (DAFF) – Importing live fish to Australia
3. DAFF – Biosecurity Import Conditions system (BICON)
4. RSPCA Australia Knowledgebase – Why water quality is important when setting up a fish aquarium (nitrogen cycle and filtration)
5. University of Florida IFAS Extension (EDIS) – Choosing an Appropriate Live Feed for Larviculture of Marine Fish (rotifers/Artemia guidance)
6. NHMRC – Australian code for the care and use of animals for scientific purposes (humane killing principles)
7. DAFF – Industry Advice 41-2025: Change to release conditions for live marine ornamental fish sharing a recirculating system
8. NHMRC – Regulation and the Australian code for the care and use of animals for scientific purposes