Carbon Dioxide in RASThe Silent Stress and How to Strip It
Why CO2 builds up in recirculating systems, what levels cause stress, and how aeration, degassing and trickle filters remove it.
EQUIPMENT REFERENCESee the mechanism.
Understand the duty.
Select a route or a stage to inspect the process. The cutaway explains the equipment and water environment; it is a teaching illustration, not a project drawing.
Measure the system
Record the test method, units and sampling location.
Use appropriate tests and record units, location, time, pH and temperature with the result.
- Record the test method, units and sampling location.
- Keep concentrations, loads and chemical forms distinct.
- Interpret results with species, pH, temperature and salinity.
Key takeaways
- Why it builds up: Fish respiration and bacterial activity both produce CO2.
- Levels to watch: Many fish and shrimp do well with CO2 kept around 15 to 20 mg/L or lower, and lethargy can show above about 50 mg/L.
- How to remove it: Expose water to air.
- The oxygen link: High CO2 makes low oxygen more dangerous because fish struggle to use what is there.
Why it builds up
Fish respiration and bacterial activity both produce CO2. Dense stocking and low turbulence make it worse. If water just sits and circulates, carbon dioxide stays dissolved.
Levels to watch
Many fish and shrimp do well with CO2 kept around 15 to 20 mg/L or lower, and lethargy can show above about 50 mg/L. Sensitive species want less.
Work with your own numbers.
Use the tools below to explore the relevant arithmetic. Enter measured values or declared assumptions. An estimate does not establish equipment capacity, biological performance or a safe operating limit.
Ideal dilution calculation
Estimate a single mixed water replacement from measured starting, replacement-water and target concentrations.
Assumptions and limits
A simple mass balance, not a safe operating target. Assumes complete mixing and no ongoing production or reaction. Species, chemistry and source-water suitability require review.
Flow & turnover planner
Relate a measured water volume to a selected turnover time. Confirm the time from the application and hydraulic design.
Assumptions and limits
A planning allowance is your assumption. This calculation does not choose a filter, prove treatment capacity or account for pump head.
Convert units before you calculate
Planning arithmetic · inputs remain visible for review · example values are not operating recommendations
How to remove it
Expose water to air. Degassing towers, trickle filters and strong aeration all strip carbon dioxide. See trickle filters.
The oxygen link
High CO2 makes low oxygen more dangerous because fish struggle to use what is there. Keep both in check. See dissolved oxygen.
Signs
Slow growth, low pH that keeps falling, and fish that seem lethargic despite good oxygen.
Pureflow Bio equipment for this
MBBR biofilters →Moving bed biofilters.Oxygen cones →Oxygen dissolving.UV disinfection systems →UV-C chambers.Make the measurements comparable
Use these questions to prepare a project brief or an operating review. Check an item only when the information has been recorded for your installation.
This checklist is a working aid in the current page session, not a saved maintenance record.
Carbon Dioxide in RAS FAQ
How is CO2 measured?
With a titration test kit or a probe made for it. Calculating it from pH and alkalinity is also common.
Do ponds have a CO2 problem?
Usually less, because water exposes more surface to air, but dense stocking in tanks can.
More guides related to carbon dioxide in ras
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