Fish need dissolved oxygen (DO) of at least 6 ppm in most ponds, and 4–15 mg/L depending on species, temperature, salinity and life stage. When stocking density goes up, aeration with air blowers often cannot keep up. Pure oxygen from an on-site PSA generator is about 93% oxygen, roughly 4.5 times richer than air, so it raises DO faster and with less electricity.
This guide explains why DO drops, how to work out how much oxygen your farm needs, and how to size an oxygen generator for a pond, raceway, RAS or hatchery.
Why dissolved oxygen matters
Dissolved oxygen is the oxygen held in the water that fish breathe through their gills. Keeping it in the right range gives:
- Better health: less stress and stronger resistance to disease.
- Faster growth and better feed conversion: fish turn more feed into body mass when they are not short of oxygen.
- Higher stocking density: more fish per unit of water, which raises output from the same farm.
- Lower mortality: fewer losses during hot weather or at night.
- Better water quality: oxygen helps bacteria break down waste, ammonia and nitrite.
Cold-water species such as salmon and trout, and broodstock and hatchery systems, are the most sensitive to low DO.
Why DO drops
- Temperature: warm water holds less oxygen, so DO falls in the hottest part of the day.
- Night-time: aquatic plants and algae produce oxygen in daylight but consume it at night, so DO is lowest before dawn.
- Feeding and waste: uneaten feed and fish waste are broken down by bacteria, which consume oxygen.
- Stocking density: more fish use more oxygen.
Measure DO regularly with a dissolved oxygen meter: put the probe in the water and read the value. Check early in the morning, when it is lowest.
Aeration vs oxygenation
Aeration blows air into the water. Air is only 20.9% oxygen, so it adds oxygen slowly and uses a lot of electricity for each kilogram dissolved.
Oxygenation injects pure oxygen, usually through fine-bubble diffusers. At 93% oxygen, each bubble carries about 4.5 times more oxygen than air, so DO rises faster and less power is spent moving gas that does not dissolve. Fine bubbles give more surface area, which improves absorption. Newer methods such as nanobubbles keep ultra-fine bubbles in the water even longer.
Most intensive farms use both: aerators to mix the water and strip carbon dioxide, and pure oxygen to hold DO at the target.
How much oxygen does a fish farm need?
The most practical way to estimate oxygen demand is from the feed. Fish consume oxygen in proportion to how much they eat.
- Daily feed: total feed given per day, in kg.
- Oxygen per kg of feed: a common planning figure is 0.3–0.5 kg of oxygen per kg of feed, depending on species and temperature.
- Daily oxygen demand = daily feed × oxygen per kg of feed.
- Allow for transfer efficiency: not all oxygen supplied dissolves. Fine-bubble diffusers in ponds may transfer roughly half; oxygen cones and contact systems in RAS transfer much more.
- Convert to generator size: 1 Nm³ of 93% oxygen carries about 1.33 kg of oxygen.
Worked example (RAS):
| Step | Value |
|---|---|
| Daily feed | 200 kg |
| Oxygen demand at 0.4 kg O₂ per kg feed | 80 kg/day, or 3.3 kg/hr |
| Oxygen to supply at 60% transfer efficiency | 5.6 kg/hr |
| Generator size (÷ 1.33 kg per Nm³) | about 4.2 Nm³/hr |
Size for the peak, usually feeding time on the warmest days, and keep an oxygen receiver to cover short spikes.
Where PSA oxygen is used in aquaculture
- Hatcheries and broodstock: stable DO for eggs, larvae and brood fish, which are the most sensitive.
- Recirculating aquaculture systems (RAS): high stocking density that depends on pure oxygen.
- Ponds and raceways: keeping DO above 6 ppm, especially at night and in summer.
- Transport and holding tanks: reducing stress when fish are moved.
- Emergency backup: oxygen on hand when aerators fail or DO crashes.
PSA generator vs oxygen cylinders and liquid oxygen
An on-site PSA generator makes oxygen from air at the farm, so there are no cylinder deliveries or refills. It costs less per kilogram of oxygen, runs 24×7, and removes the risk of running out on a hot night. PSA systems work for both freshwater and marine farms, using corrosion-resistant materials where needed.
For the full engineering of how a PSA generator works, read the PSA oxygen generator guide.
Safety
Install the generator and pipework correctly, leak-test the oxygen lines, monitor oxygen purity and pressure, and train staff to handle oxygen safely.
Get an oxygen system sized for your farm
See our oxygen generators for aquaculture and fish farming, or the compact Idos C for smaller farms. Email info@absstem.com or call 1800 547 0506 for help with DO targets and sizing.