Oxygenation for biofloc and RAS fish farms

Why oxygen is the limit in biofloc and RAS

Biofloc and recirculating aquaculture systems hold far more fish in a given volume of water than an earthen pond. That density is the point of the system, and it is also why dissolved oxygen becomes the first limit. The fish respire around the clock, and the feed that drives their growth adds a further oxygen demand as it is eaten and digested.

In biofloc, the floc itself is a dense community of bacteria that turns ammonia and waste into microbial protein. Those bacteria need oxygen too, so a biofloc tank carries two oxygen loads at once: the fish and the floc. In a RAS, the biofilter plays the same role, and its nitrifying bacteria draw on the same oxygen as the fish.

Demand is not steady. It rises after each feed, as the fish digest and the floc or biofilter processes the waste, and it continues through the night. Dissolved oxygen is usually lowest in the early morning. A reading taken at midday can look comfortable while the tank is close to its limit before dawn, which is why morning readings tell you more than any single daytime value.

When oxygen runs short, fish eat less and convert feed less efficiently, ammonia conversion slows, and in the worst case stock is lost overnight. Holding oxygen steady through the feeding and night-time dips is the job of an oxygenation system.

Where a unit fits in a biofloc tank or RAS loop

A nanobubble unit runs as a side-stream. It draws water from the culture tank or, in a RAS, from the sump, passes it through the generator and returns it carrying gas as bubbles under 200 nm. Because the bubbles stay in the water rather than rising straight to the surface, more of the gas has time to dissolve.

The unit runs alongside the blowers or diffusers you already have. It does not replace them: in a biofloc tank the existing aeration also keeps the floc in suspension, and in a RAS the existing system is designed for the whole loop. What a nanobubble unit adds is a steadier supply of dissolved oxygen through the feeding and night-time dips.

In the Hajo trial, the nanobubble tanks were served by an air-fed 10 LPM unit, which needs no oxygen supply. The right unit for your system depends on tank volume, stocking and your dissolved-oxygen target, and we confirm it after a site visit. See the nanobubble generator range.

What to measure in a pilot

• Dissolved oxygen, morning and evening.
• Ammonia, nitrite and nitrate.
• pH and temperature.
• Survival.
• Feed supplied, and feed conversion ratio.
• Power use, metered at the unit.
• A matched control tank with the same aeration hours and water exchange.

Why each measurement matters

Morning dissolved oxygen shows how close the system comes to its limit, and evening readings show how much margin it carries into the night. Ammonia, nitrite and nitrate show whether the floc or biofilter is keeping pace with the feed going in. Survival and feed records turn water quality into the figures a farm is actually run on. Metered power gives a running cost based on what the unit draws in service, not on its nameplate.

The matched control is what makes the result usable. Without it, a better crop could come from the season, the stocking or the feed rather than the oxygen. With these records, a pilot answers the question the Hajo trial could not: what the nanobubble unit adds when everything else is held the same.