How much CO2 is in aquarium water without injection?
Short answer
Water in equilibrium with outdoor air holds about 0.65 mg/L of CO2 at 25 °C, 0.74 mg/L at 20 °C and 0.57 mg/L at 30 °C, from the Henry's law constant compiled in the NIST WebBook and NOAA's September 2026 Mauna Loa mean of 426 ppm [1, 2]. Tropica gives the natural level in an aquarium as 0–0.5 mg/L [4]. Either way it is a small fraction of the 10–30 mg/L that growers recommend for demanding plants.
Henry’s law
The amount of a gas that dissolves in water at equilibrium is proportional to the gas’s partial pressure above the water. For CO2 the proportionality constant at 25 °C is about 0.034 mol per kilogram of water per bar, in the compilation of Henry’s law constants published in the NIST Chemistry WebBook [1]:
dissolved CO2 (mol/kg) = kH × pCO2 (bar)
The partial pressure of CO2 in air is its mole fraction times the air pressure. NOAA’s monthly mean at Mauna Loa for September 2026 was 426.07 ppm [2]. At sea-level pressure (1.013 bar) that is about 0.000432 bar.
The calculation
At 25 °C:
- 0.034 × 0.000432 = 1.47 × 10⁻⁵ mol/kg
- × 44.009 g/mol [3] = 0.65 mg per kilogram, which is effectively 0.65 mg/L
The constant changes with temperature. The WebBook gives a temperature dependence of about 2400 K, meaning kH(T) = kH(25 °C) × exp(2400 × (1/T − 1/298.15)) [1]. Applying it:
| Water temperature | kH (mol/kg/bar) | CO2 at equilibrium |
|---|---|---|
| 20 °C | 0.039 | 0.74 mg/L |
| 25 °C | 0.034 | 0.65 mg/L |
| 28 °C | 0.031 | 0.60 mg/L |
| 30 °C | 0.030 | 0.57 mg/L |
The WebBook lists k°H values mostly of 0.034–0.035 (with outliers from 0.031 to 0.045) and temperature terms from 2300 to 2700 K, most often 2400 K, from different studies [1]; using the other common values shifts these results by a few percent. The simplification here also ignores the small reduction in partial pressure from water vapor in the air.
Compared with grower targets
Tropica gives the natural CO2 level in an aquarium as 0–0.5 mg/L [4], close to this calculation. Its guide says that without added CO2, plants make do with what fish and bacteria release, which it calls inadequate for its Medium and Advanced plants [5], and its CO2 system page suggests approximately 3 mg/L for tanks with only Easy plants, 10 mg/L for Medium and 25 mg/L for Advanced [4]. Even the lowest of those is about five times the air-equilibrium level.
Equilibrium pH
The same equilibrium sets the pH that tank water drifts toward when CO2 is not injected. With CO2 at 0.65 mg/L and the carbonate constant at 25 °C [6], water of 2 dKH settles near pH 8.0, 4 dKH near pH 8.3 and 6 dKH near pH 8.5, before any effect of other buffers. That baseline is what the pH-drop method compares against.
What this means in practice
- Without injection, CO2 is set by gas exchange with the air plus whatever biology adds. It cannot be raised by much without a CO2 source, which is why growers sort plants into categories by their CO2 needs.
- With injection, the tank runs far above equilibrium, and any surface agitation works to pull it back toward 0.6–0.7 mg/L. That is one reason injected tanks need a steady supply rather than a one-off dose.
- Temperature has a modest effect on the equilibrium value: about a quarter less CO2 at 30 °C than at 20 °C.
Questions
Does surface agitation remove CO2?
Agitation speeds up gas exchange, which pulls dissolved CO2 toward the air-equilibrium value in either direction: down if the tank is above it, up if plants have drawn it below. The equilibrium value itself is set by the air, not by the agitation.
Why does warm water hold less CO2?
Gas solubility falls as temperature rises; the NIST WebBook expresses this as a temperature dependence of the Henry's law constant of about 2400 K for CO2 [1].
Calculators for this
Related guides
- CO2 & lightCO2 from KH and pH: the formula, the chart and where it breaksWhere the KH/pH/CO2 chart comes from, why its factor of 3 differs from the temperature-corrected 3.5, and the conditions under which both estimates fail.
- CO2 & lightLiquid carbon products: what the labels claim and what is establishedWhat Seachem Flourish Excel and Tropica Carbon Nutrition say about themselves, what their pages leave out, and the hazard data PubChem lists for glutaraldehyde.
- CO2 & lightPressurized CO2 systems: the parts and what each one doesCylinder, regulator, needle valve, solenoid, bubble counter, check valve and diffuser: what each part does, per manufacturers, plus the gas safety basics.
- CO2 & lightSetting CO2 with the pH-drop methodCompare tank water with a degassed sample: each pH unit of difference means ten times the CO2. The arithmetic, what the baseline really is, and its limits.
Sources
- Carbon dioxide: Henry's Law data. NIST Chemistry WebBook, SRD 69 (compiled by R. Sander). k°H 0.034–0.035 mol/(kg·bar) at 298.15 K; d(ln kH)/d(1/T) 2300–2600 K; formula kH(T) = k°H exp(d(ln kH)/d(1/T) × (1/T − 1/298.15)). Accessed October 12, 2026.
- Trends in Atmospheric Carbon Dioxide (Mauna Loa). NOAA Global Monitoring Laboratory. Monthly average September 2026: 426.07 ppm (dry-air mole fraction); September 2025: 424.37 ppm; updated 9 October 2026. Accessed October 12, 2026.
- Carbon dioxide (CID 280). PubChem, National Library of Medicine. Molecular weight 44.009 g/mol. Accessed October 12, 2026.
- CO2 System Nano. Tropica Aquarium Plants. Natural CO2 level 0–0.5 mg/L; even a slight addition helps; approx. 3, 10 and 25 mg/L for Easy, Medium and Advanced plants. Accessed October 12, 2026.
- Fertiliser and CO2. Tropica Aquarium Plants. CO2 is the main nutrient and main growth limiter; without added CO2 plants rely on what fish and bacteria release, inadequate for Medium and Advanced plants. Accessed October 12, 2026.
- The solubilities of calcite, aragonite and vaterite in CO2-H2O solutions between 0 and 90 °C, and an evaluation of the aqueous model for the system CaCO3-CO2-H2O. Geochimica et Cosmochimica Acta 46: 1011–1040. Plummer, L.N. and Busenberg, E. (1982). Carbonate equilibrium constant used for the equilibrium pH estimate. Accessed October 12, 2026.