Why CO2 Is Often Missed
Dissolved oxygen gets most of the attention in RAS, but elevated carbon dioxide can quietly reduce feed intake, depress pH, impair oxygen uptake, and stress fish. A tank can show acceptable DO while fish are still struggling because CO2 has not been stripped effectively.
Main CO2 Sources
CO2 comes from fish respiration, microbial activity, and organic solids decomposition. Poor solids removal increases CO2 load because waste breaks down inside the loop. Biofilters also contribute to CO2 and alkalinity consumption.
Design Priorities
CO2 control should combine source reduction and gas stripping.
- Remove solids quickly before they decompose
- Use degassing towers, packed columns, trickling filters, weirs, or splash points where suitable
- Keep pumps after degassing points when the hydraulic layout allows it
- Avoid unnecessary pressure that keeps CO2 dissolved
- Maintain alkalinity so pH does not crash as CO2 rises
Low-Cost CO2 Reduction Methods
Not every system needs an expensive forced-air degassing tower at every stage. Early design can use passive physics effectively.
- Surface agitation at tank returns
- Cascading weirs or open-channel returns
- Spray bars over sumps or tanks
- Wide shallow sumps with good turbulence
- Simple packed columns with low-pressure airflow
- Trickling filters where compatible with the overall treatment concept
These methods are most effective when paired with strong solids capture and clean hydraulics.
What to Avoid
- Treating oxygen injection as a substitute for CO2 stripping
- Relying only on pH dosing while ignoring the underlying CO2 load
- Placing high-pressure pumping before the main degassing stage
- Letting biofilters accumulate solids and anaerobic pockets
- Waiting for fish stress before testing CO2
Operating Checks
Operators should compare DO, pH, alkalinity, feeding behavior, tank flow, and degasser performance together. If feed intake drops while DO looks acceptable, CO2 should be part of the first troubleshooting path.