Fry Grow-Out Tanks: Layout, Filtration, Density and Sorting
What research on larval and juvenile fish shows about grow-out tanks: barren vs structured tanks, sponge filters, water quality, stocking density effects on growth, and size grading.
What a grow-out tank is for
A grow-out tank holds fry from the point where they swim and feed independently until they are large enough to join adults or be rehomed. Hatching, first foods and early care are covered in raising-fry-guide and feeding-fry-guide; this guide deals with the tank itself. Extension material on ornamental fish production stresses that under tank conditions water quality is the most critical factor, because large numbers of fish are held in small volumes, and that each tank behaves as an isolated population for disease management.
Zebrafish husbandry recommendations illustrate the typical sequence: embryos settle on the bottom until about 5 days post-fertilisation (dpf), when the swim bladder fills; early larvae are often kept in small tanks with the water flow turned off; and juveniles are moved to aquaria with water exchange once they reach a suitable size.
Barren or structured tank
Most laboratory zebrafish are reared and kept in barren tanks. A 2026 facility study found that adding artificial plants had neither detrimental nor beneficial effects on husbandry outcomes but reduced the available swimming space. A 2023 study testing a gravel picture, a plastic plant or both found no main effect of enrichment, although fish reared with enrichment showed less variation in body length at some densities. The authors note that studies reporting benefits of enrichment used more complex structure, including real gravel substrate.
No controlled study on bare-bottom versus planted grow-out tanks for aquarium species was found, so the choice is a husbandry trade-off between space, structure and ease of keeping water quality high, rather than an evidence-based rule.
Filtration
A sponge filter is driven by an air pump: rising bubbles in a tube draw water through a porous foam that hosts nitrifying bacteria. Wikipedia describes it as especially useful for rearing fry, because the sponge stops small fish from entering the filter, and notes that such filters suit small, lightly stocked aquaria. Undergravel filters can clog with fine substrates such as sand.
Water changes and growth
Practical Fishkeeping reports some evidence, mainly from fish-farming research on cyprinids and salmonids, that chemicals released by fish can suppress the growth of their own species, while other groups such as tilapia and catfish appear unaffected. The article treats the effect cautiously and concludes that adequate tank size and regular water changes matter more for growth than chemical filtration, adding that overcrowding and high nitrate also impair health. No peer-reviewed study fetched for this guide gives a water-change percentage or frequency for ornamental fry, so no figure is given here.
Stocking density
Density figures are species-specific and come from controlled trials:
- Giant gourami (Osphronemus goramy), larvae from 8 days post-hatch, 21 days in a recirculating system with food always available: survival exceeded 98 % at all densities from 0.6 to 19.2 larvae per litre, but growth fell steadily with density. Final mean weight was about 563 mg at 0.6 larvae/L and about 82 mg at 19.2 larvae/L, probably because food intake dropped at the highest densities.
- Zebrafish (Danio rerio): recommendations allow up to 250 larvae per litre at 5–10 dpf and 4–10 adults per litre for long-term housing, noting that body mass rather than number sets the limit, so juveniles can be held more densely than adults.
- Zebrafish raised at 5, 25 or 50 fish per 2.5 L tank on the same ration: the low-density fish grew faster and formed scales earlier (7 scale rows at 4.5 weeks versus up to 6.5 weeks at high density).
- Zebrafish kept at 1, 3 or 6 fish per litre: fish at 1 fish/L grew longer and heavier but were significantly more aggressive and showed higher group cortisol; the authors suggest 3–6 fish/L as the likely optimum.
Together these results show that lower density usually means faster individual growth, but very low density can increase aggression in social species, so the target depends on the species and on how much food and water quality the system can sustain.
Sorting by size
UF/IFAS describes grading as separating fish into size classes, using graders with bar spacing measured in millimetres, and judging quality by colour, fin development, sex and health signs such as scale loss, torn fins, eye damage and parasites. Careful handling during grading is important to limit stress.
In a study on larval and juvenile pike (Esox lucius), a piscivorous species, grading halved cannibalism among the large fish compared with small and ungraded groups. Across all graded fish, however, grading did not improve overall survival or biomass gain compared with ungraded fish. Sorting is therefore most useful where cannibalism is the main loss, and its effect for a given aquarium species should not be assumed.
Diet changes also shape the grow-out period. In Betta splendens a functional stomach appears between 15 and 18 days post-hatch, and larvae could switch from live nauplii to a microparticulate diet from 15 days without losing growth or survival, while microdiet from first feeding gave lower survival. Details are in feeding-fry-guide.