Why are fish fry dying in the first week?

The first week contains several different risk stages

Newly hatched fish are not simply miniature adults. Depending on the species, they may spend a period relying on yolk reserves before they begin external feeding. Their mouth, swimming ability and digestive system are still developing, and the transition to first food can be a major bottleneck. Treating all deaths in the first week as one problem hides useful timing information.

Write down when the first hatch occurred, when fry became free-swimming, when the first food was offered and when deaths increased. A loss peak before external feeding points you toward incubation, development and water conditions; a peak after first feeding makes food size, food availability and fouling more important suspects.

Check water quality before changing the diet

Small fry containers can deteriorate rapidly. Heavy feeding, dead eggs, empty shells and weak filtration create waste while the fry themselves tolerate little instability. Test ammonia and nitrite and verify temperature. Observe respiration and where the fry sit in the water column. Surface crowding or weak, uncoordinated behaviour can indicate an urgent environmental problem even before a test confirms it.

• Remove dead fry, shells and uneaten food without creating strong suction.
• Use a sponge guard or fry-safe filtration so the intake is not a mortality source.
• Keep oxygenation adequate but avoid current that exhausts tiny fry.
• Match replacement water carefully during frequent small maintenance.
• Do not assume a clear-looking nursery has safe ammonia and nitrite.

First food must fit the fry

At first feeding, mouth size can physically limit what the fry can swallow. A food can be nutritionally excellent and still be useless if it is too large, too fast or rarely encountered. Some species take prepared microdiets readily; others perform much better on appropriately sized live food. The correct choice depends on species and developmental stage, not on a universal “fry food” label.

Offer small amounts often enough for fry to encounter food while keeping the nursery clean. A cloud of excess powder is not proof that fry are eating; it may simply become bacterial load. Observe actual strikes and full bellies where visible.

A first-week triage sequence

1. 1. Test the nurseryCheck ammonia, nitrite, temperature and obvious oxygen or flow problems before adding more food or medication.
2. 2. Confirm developmental stageDetermine whether the fry are still using yolk reserves or have begun active external feeding.
3. 3. Watch actual feedingOffer a species-appropriate particle or live-food size and observe whether fry can capture and swallow it.
4. 4. Reduce mechanical lossesProtect intakes, siphon gently and remove debris without trapping fry against nets or filters.

Do not medicate a nursery blindly

Rapid fry loss can tempt repeated medication, but fry are small, sensitive and difficult to dose safely. If the primary problem is ammonia, oxygen, temperature or starvation, medication adds stress without fixing the cause. Disease becomes more plausible when water and feeding are demonstrably appropriate yet losses continue with lesions, abnormal swimming or a repeated pattern across batches.

Uneven loss can reveal the mechanism

If the smallest fry disappear first, food access or size competition may be involved. If fry near an intake vanish, investigate mechanical loss. If an entire batch declines after a maintenance event, review replacement water, temperature and chemical exposure. If multiple unrelated tanks are affected, stop sharing nets, pipettes and water until cross-contamination is ruled out.

Use the next batch to test one improvement

Keep the breeding container, water source and parent group as consistent as practical, then change the factor with the strongest evidence—such as first food size, intake protection or waste removal. Record daily survival estimates, feeding observations and water results. One well-documented batch teaches more than several heavily altered batches where every variable changed at once.

When fry losses are severe despite stable measured conditions and appropriate feeding, or when deformity and abnormal behaviour are widespread, seek specialist aquatic-health or species-specific hatchery guidance.