Why are some fish fry not growing?

Decide whether the size gap is actually increasing

Fry from the same spawn do not grow at exactly the same rate. Eggs may hatch at different times and individual feeding success varies. The useful warning sign is not simply that one fish is larger; it is that the distribution becomes more uneven over time and the smaller group loses condition or stops progressing.

Photograph a representative group from above or against a scale at regular intervals. Do not repeatedly net every fry just to measure it. A consistent visual record can show whether the whole batch is growing, whether only the leaders are accelerating, or whether a subset has stalled.

Food access is often more important than total food offered

A tank can receive plenty of food while smaller fry still go hungry. Larger, faster juveniles can monopolise concentrated feeding points, and food particles may quickly become too large for lagging fry. Feed distribution across the nursery and particle size therefore matter alongside total ration.

• Watch whether the smallest fry make successful feeding strikes.
• Offer an appropriate range of particle sizes as the batch transitions between foods.
• Spread food across more than one feeding area when social competition is obvious.
• Remove excess food so solving competition does not create a water-quality problem.
• Check that flow does not sweep food away from weak or bottom-oriented fry.

Density changes the whole system

As fry grow, the same number of fish produces more waste, consumes more oxygen and competes for more space. A nursery that was suitable at hatch can become crowded surprisingly quickly. Uneven growth is sometimes the first visible sign that the grow-out plan has not kept pace with biomass.

Use actual growth and water-test trends to decide when to split a batch. Do not rely only on a fixed “fish per litre” rule borrowed from another species or filtration system.

When grading helps

Grading means separating juveniles into reasonably similar size groups. It can improve feeding access and reduce predation or bullying in species where large siblings attack smaller ones. It also adds handling stress and requires extra cycled space. Grade when the benefit is clear, not as a cosmetic attempt to make every juvenile identical.

If cannibalism is possible, the threshold for separating large individuals may be lower. If the species is delicate and peaceful, improving food distribution and space may be safer than frequent capture.

A practical growth audit

1. 1. Compare appetiteObserve successful feeding in both large and small fry rather than judging only how much food enters the tank.
2. 2. Review density and waterCheck ammonia, nitrite, oxygenation, flow and maintenance as biomass increases.
3. 3. Inspect the small groupLook for deformity, emaciation, abnormal waste, lesions or behaviour that suggests disease rather than simple competition.
4. 4. Separate only when usefulSplit size classes if larger juveniles are monopolising food, injuring smaller fish or creating a persistent widening gap.

Do not chase growth with overfeeding

Adding more food indiscriminately can worsen the conditions that suppress growth. Excess feed increases organic waste and can destabilise a small grow-out system. Growth depends on food being eaten and converted, not on the amount left on the bottom. Increase feeding only when observation and maintenance capacity support it.

Record growth as a husbandry outcome

Keep notes on hatch date, first food, feed changes, estimated density, splits, water tests and approximate size range. If the same parent combination repeatedly produces a small non-growing subgroup despite good feeding and environment, developmental or genetic factors become more plausible. Do not cull or medicate based on size alone without evidence of a welfare problem.

Persistent failure to grow with appetite loss, abnormal body shape, chronic wasting or repeated deaths deserves qualified diagnostic advice. A growth problem can be an early sign of disease, but size difference by itself is not a diagnosis.

Judge growth as a trend, not a single snapshot

A size spread becomes more useful when it is measured. Photograph a representative group against the same scale each week or record lengths from a small, gently handled sample. Compare appetite, body shape and activity as well as length. A small fry that is steadily growing may simply be behind its siblings, while a fry that stops growing, becomes thin or loses interest in food deserves closer attention. The goal of grading is to improve access and safety, not to remove every natural difference.

When separating size classes, avoid creating a “small fry” tank that is less stable than the main grow-out system. Match temperature and water chemistry, use mature biological filtration or another proven safe method, and keep feeding measurable. If the smallest fish only fall further behind after separation, reassess food type, feeding access, health and water quality rather than assuming genetics explains everything.