How many fish can it hold?
Stocking density is the weight of fish per volume of water, given in kilograms per cubic metre (kg/m³) or pounds per gallon. For a tank that is strongly aerated but gets no pure oxygen, the rule of thumb of the Southern Regional Aquaculture Center (SRAC) is a maximum of about 30 kg/m³ (0.25 lb per US gallon) at harvest: 30 kg of fish in 1,000 litres (SRAC 282). The calculator works back from that ceiling; its two gentler settings are our own safety margins of a half and a third of it. When in doubt, stock under it.
Start lower. This is a ceiling for a mature, well-run system - not a target for day one. While you learn, stock at half this and let steady water quality tell you when to add more.
For tanks, IBC totes and small lined ponds that have a filter or an aerator - not for an unaerated earthen pond, which carries far fewer fish because its limit is set by surface area and oxygen, not by volume. A rule of thumb, not a guarantee. Real carrying capacity depends on temperature, feeding, water changes and the strength of your filtration and aeration - a warm, still system holds far fewer fish than a cool, heavily aerated one. Watch your water test kit, not just this number.
Stocking density: common questions
How many fish can 1,000 litres hold?
At the SRAC ceiling of about 30 kg/m³, 1,000 litres (264 US gallons) of strongly aerated tank water carries about 30 kg of fish at harvest: roughly 44 tilapia of 680 g (1.5 lb) each, the generally accepted size for the live tilapia market (SRAC 282). Half of that, about 22 fish, is the calculator's default while you learn the system.
Can a tank hold more fish with pure oxygen?
In commercial systems, yes. SRAC 282 says that where supplemental oxygen is added and vigorous aeration is not used, maximum density can often exceed 60 kg/m³ (0.5 lb per gallon), and some designs run at 90-120 kg/m³ (0.75-1.0 lb per gallon). The same sheet warns that such densities leave the fish exposed to any power cut or equipment failure without backup systems and alarms, and that efficient solids removal is the key to keeping the water safe.
How dense can trout be kept?
Trout farms in flowing raceways use a density index: keep the fish weight in pounds per cubic foot at 0.5 to 1 times the fish's length in inches, so 4-inch (10 cm) trout up to 2-4 lb per cubic foot (32-64 kg/m³). Many growers cap grow-out at 4.5 lb per cubic foot, about 72 kg/m³ (SRAC 222). Those figures assume a steady flow of fresh stream water; a closed home tank is held to the lower ceiling above.
What weight do farmed fish reach?
The weight of one fish sets how many fit under the ceiling. SRAC gives about 680 g (1.5 lb) as the generally accepted size for the live tilapia market, with 340-908 g (0.75-2 lb) fish accepted in some areas, and notes that females grown together with males reach only about half the males' size (SRAC 282); most farm-raised channel catfish are harvested at about 570 g (1.25 lb) at around 18 months (SRAC 180); red claw crayfish reach 50-100 g (2-4 oz) in a seven-month warm season (SRAC 244). The other starting weights in the calculator are examples: put in the weight you plan to harvest.
Where next?
The species guides for tilapia and channel catfish cover feeding and harvest, and systems and water quality covers aeration, filters and test kits.
Kit for raising fish
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