Recirculating Aquaculture Systems (RAS)

Ozone generators improve water quality in fish farms and Recirculating Aquaculture Systems. Also known as RAS, these indoor tank systems filter and reuse the same water. They reduce harmful bacteria, break down dissolved organic waste, and increase dissolved oxygen. At the same time, they do not leave chemical residue in the water. A2Z Ozone has supplied ozone systems to fish farmers across the Midwest and beyond. Our expertise in ozone goes back 24 years.

A Recirculating Aquaculture System, or RAS, is an indoor fish farming setup. The water is continuously filtered and reused. So instead of drawing constantly from a river, pond, or well, an RAS recirculates 90–95% of its water volume.

The water cycles back through the tanks after special filtering and treatment. This makes an RAS popular for raising high-value fish species, including bass, tilapia, trout, and shrimp. It allows farmers to breed fish in locations far from natural habitats. That is how farmers can raise these fish even in the Midwest United States.

Reusing water means that dissolved waste, pathogens, and organic matter accumulate quickly. Keeping that water clean for healthy fish is the central challenge of any RAS operation. Ozone is one of the most effective tools available for that job.

What Ozone Does in a Fish Farm or RAS

When applying ozone to the water supply of a fish farm or RAS, it works on several problems at once.

It destroys bacteria and pathogens.

 

 

 

 

Ozone attacks the cell walls and internal structures of bacteria and pathogens. Research published in Frontiers in Microbiology (Yousef et al., 2024) shows that when ozone is dissolved directly into water, it can effectively kill harmful bacteria such as E. coli, Listeria, Salmonella, and Pseudomonas aeruginosa.

Even at doses that do not harm the living fish, ozone works its wonders. A study using tilapia found that ozone nanobubble treatments reduced two major fish pathogens, Streptococcus agalactiae and Aeromonas veronii, by 99.93% to 99.99% after repeated treatments. At properly controlled levels, no harm came to the fish.

 

 

 

 

 

 

 

 

It clears dissolved organic matter (DOM) that builds up.

In a recirculating system, dissolved organic matter accumulates. Fish waste and feed residue float in the water even after it’s filtered. This cloudiness stresses fish and provides food for harmful bacteria. A 2023 study in Environmental Monitoring and Assessment found that using ozone in RAS improved water quality. It removed over 90% of the organic material that makes the water dirty. In fact, the systems that used only ozone worked better than those that used hydrogen peroxide or ozone and hydrogen peroxide together.

 

 

It improves dissolved oxygen levels.

Ozone breaks down quickly in water. That process is actually one of its great advantages in aquaculture. As it decomposes, it reverts to ordinary oxygen. In this manner, it boosts dissolved oxygen levels in the tank water. Higher dissolved oxygen supports faster fish growth rates and reduces stress and mortality.

It leaves no chemical residue.

Unlike chlorine-based disinfectants, ozone leaves no lasting chemical in the water after it does its work. There is nothing to flush out before the water returns to the fish.

 

What to Expect with Ozone in a RAS (Important Safety Notes)

Ozone is a powerful oxidizer, which means it must be used correctly. Too much dissolved ozone in contact with fish can damage gill tissue. This is not a reason to avoid ozone. It is important to size and control the system.

Targets for Residual Dissolved Ozone

Fish farmers keep the target for residual dissolved ozone in a RAS environment very low. To protect their fish, they keep levels in the range of 0.01 to 0.05 mg/L. They measure these levels after the contact/reaction chamber and before the water returns to the fish tanks. The actual treatment happens in a dedicated contact tank. That allows ozone to break down before the treated water re-enters the environment.

Which A2Z Ozone Unit Is Right for a RAS?

The correct ozone generator size depends on your total water volume, flow rate, organic load, and fish species. We recommend using a timer. One hour on and two hours off may be perfect for your system.

As a general starting point:

System Size	Typical Water Volume	Suggested Ozone Generator*
Small indoor farm	Up to 2,000 gallons	Z-7G (up to 3 g/hr output w/oxygen source)
Mid-size RAS	2,000–8,000 gallons	Z-10G (up to 7–10 g/hr w/oxygen)
Large commercial RAS	8,000+ gallons	S-Series or custom sizing (S-16G to S-100G) Call for assistance

*Based on achieving levels of 0.01 to 0.05 mg/L.

For help, call Eng. Albert Noroozi, Monday through Friday, 9 am to 3 pm Eastern time.

 

Sources

Yousef, A.E. et al. (2024). Inactivation kinetics of selected pathogenic and non-pathogenic bacteria by aqueous ozone. Frontiers in Microbiology, 14. https://pmc.ncbi.nlm.nih.gov/articles/PMC10829095/

Jhunkeaw, C. et al. (2020). Ozone nanobubble treatment in freshwater effectively reduced pathogenic fish bacteria and is safe for Nile tilapia (Oreochromis niloticus). Aquaculture, 534. https://linkinghub.elsevier.com/retrieve/pii/S0044848620339922

Pettersson, S. et al. (2023). Monitoring of water quality with HPLSEC and fluorescence method in the ozonated recirculating aquaculture system. Environmental Monitoring and Assessment, 195. https://link.springer.com/10.1007/s10661-023-12117-5

Aguilar-Alarcón, P. et al. (2022). Impact of ozone treatment on dissolved organic matter in land-based recirculating aquaculture systems. Science of the Total Environment, 842. https://linkinghub.elsevier.com/retrieve/pii/S0048969722041067