Carbon Dioxide (CO2)

For farmers, designers or anyone understanding intensive systems: carbon dioxide (CO₂) forms when fish and microbes respire - roughly the mirror image of oxygen use. Fish take up O₂ and release CO₂ into the water. In open ponds and cages, CO₂ is often diluted by large water volumes and gas exchange with air. In RAS, semi-flow and other high-density, low-exchange systems, dissolved CO₂ can rise until it limits growth and welfare - sometimes while oxygen still looks "fine" on the display.

That is why CO₂ is one of the parameters that separate "enough oxygen" from "fish that actually perform". Modern intensive culture must design for both oxygen supply and carbon dioxide stripping - see also the dissolved oxygen (DO) section.

What does CO₂ do to the fish?

Dissolved CO₂ affects blood and respiration. High levels lower blood pH (respiratory acidosis), make it harder to load and unload oxygen via haemoglobin systems, and cause stress even when DO in mg/L looks acceptable. Fish may increase ventilation rate, reduce feed intake, lose growth and become more disease-prone. At strongly elevated CO₂ you see lethargy, crowding at the surface or inlet, and in the worst case mortality - often combined with other stressors (heat, low alkalinity, handling).

Tolerance depends on species, life stage, acclimation and how fast CO₂ rises. Salmonids in intensive land-based culture are often held to fairly low targets in literature and practice (many farms aim on the order of single-digit to low tens of mg/L free CO₂ depending on species and policy) - exact limits should be set with species knowledge and veterinary/production standards, not web rules of thumb alone.

Chemistry: CO₂, pH and alkalinity

In water, CO₂ is part of the carbonate system: dissolved CO₂ (roughly H₂CO₃*) ↔ HCO₃⁻ ↔ CO₃²⁻. When more CO₂ dissolves, the equilibrium shifts, pH falls and the fraction of "free CO₂" rises. Alkalinity (buffer, often measured as HCO₃⁻ equivalents) dampens pH swings: at low alkalinity the same CO₂ addition causes a larger pH drop and more stress. So CO₂, pH and alkalinity belong together - see also the pH and alkalinity sections. In freshwater RAS, bicarbonate/lime make-up may be needed both for the biofilter and to hold buffer when CO₂ is stripped and nitrification consumes alkalinity.

Important: pH alone is not the same as CO₂. The same pH can mean different free CO₂ depending on alkalinity and temperature. So it is rarely enough to "control on pH" if you want to know whether fish are breathing in a CO₂-loaded environment - you need either measured/calculated free CO₂ or a calibrated model against alkalinity and pH.

Where does CO₂ come from?

• Fish respiration - rises with feed, biomass, activity and temperature.
• Microbes and biofilters - heterotrophic and nitrifying activity also produce CO₂.
• Breakdown of organics (feed fines, faeces) in systems with high organic load.
• In ponds: night-time respiration from plankton and sediment can raise CO₂ while DO falls.

As with oxygen, peak feed is the sizing load: more feed → more O₂ demand and more CO₂ production at roughly the same time. Summer operation (warmer water, higher metabolism) is often critical for both gases. For salmonids, early design often uses roughly 250-400 g CO₂ per kg feed as an order of magnitude (same uncertainty as for O₂) - see mass balance and emission profile under dissolved oxygen (DO).

Why CO₂ is often the bottleneck before oxygen in RAS

Pure oxygen and cones/LHOs can raise inlet DO quickly. Removing CO₂, by contrast, needs gas exchange against a gas phase with low CO₂ partial pressure - typically air in a degasser, cascade or packed column - plus enough contact time and area. If you only "pump in oxygen" without stripping CO₂, you can create a farm with high DO and still reduced feed intake and growth. Many high-intensity systems are therefore CO₂-limited: degasser size, air flow, head and hydraulics set the ceiling, not oxygen gas flow alone.

Stripping and technology choices

• Degasser / CO₂ stripper - cascade, packed bed or similar; air against water, often after the biofilter.
• Aeration in tank or sump - simpler, lower efficiency; more common in semi-flow and ponds (paddlewheels etc.).
• Trickling filters / aerated biofilters - can provide both nitrification and some gas exchange.
• Water exchange / make-up - dilutes CO₂ but costs water and heat; complements, rarely replaces a degasser in high-reuse RAS.

Best practice in many RAS designs: maximise air-side gas exchange in the degasser (CO₂ out, O₂ in from air), then add pure oxygen to reach the desired total ΔDO - do not replace all aeration with pressurised O₂ alone. That often cuts energy need and the risk of forgetting CO₂. See degassers, LHO, oxygen cones and oxygen demand under equipment and oxygen & gas.

Measuring CO₂

CO₂ is harder to "just put a probe on" than temperature. Common paths: (1) calculate free CO₂ from pH, alkalinity, temperature and salinity with carbonate equilibria - needs reliable alkalinity and pH measurement; (2) direct CO₂ sensors / membrane or NDIR solutions in process water where available and maintained; (3) lab or field kits as spot checks. Online pH is often used as a fast indicator, but as noted above pH is not identical to CO₂ without known buffer.

Practical problems: wrong alkalinity → wrong calculated CO₂; pH electrodes that drift or foul; slow response; measurement in a sump that does not represent the tank; and staff trusting DO alarms while CO₂ quietly rises at peak feed. On high-intensity farms, CO₂ (measured or calculated) should be part of the operating picture together with DO, pH and feed - see monitoring systems.

Design rules

• Design oxygen and CO₂ together - size degasser/air for peak feed, not oxygen gas alone.
• Track alkalinity and pH; low buffer amplifies CO₂ problems.
• Avoid "fixing" low DO with more O₂ alone if fish show CO₂ stress (poor appetite, high ventilation).
• Ensure air flow, head and maintenance on the degasser - fouling and bad air cause quiet CO₂ rise.
• Measure or calculate free CO₂ as needed; do not rely only on green DO.

In short: oxygen is the fuel, but CO₂ is the exhaust. An engine that only gets fuel without an exhaust outlet runs poorly - the same logic applies to intensive fish farming. Good gas balance is O₂ in, CO₂ out, stable buffer and hydraulics that mix the tank evenly.