Free pH KH CO2 Calculator - Measure Dissolved CO2 | Fish Tank Calculator

Calculate actual dissolved CO2 ppm from your aquarium's pH and KH readings. Verify CO2 injection is in the safe 15–30 ppm range for planted tanks.

FAQ

How is dissolved CO2 calculated from pH and KH?

The formula is: CO2 (ppm) = 3 × KH (°dKH) × 10^(7 − pH). This is derived from the carbonic acid equilibrium in freshwater. Higher pH or lower KH means less dissolved CO2. The formula assumes a closed system with no other acids present.

What is the safe CO2 range for planted tanks?

The ideal range is 15–30 ppm. Below 15 ppm, plants may not grow well. Above 30 ppm, fish can show signs of stress such as gasping at the surface. Monitor fish behavior and adjust CO2 injection accordingly.

How do I measure KH?

KH (carbonate hardness) is measured with a standard API KH test kit or digital meter. It is expressed in °dKH (German degrees) or ppm CaCO3. 1 °dKH = 17.848 ppm CaCO3.

Why does my pH test show high CO2 if I'm not injecting?

Tap water, organic decomposition, fish respiration, and substrate all contribute to CO2. Even without injection, tanks typically have 2–5 ppm of dissolved CO2. This calculator shows what is currently dissolved, not what you've injected.

Does this formula work for saltwater tanks?

No. Saltwater chemistry is more complex due to borate alkalinity. This formula applies only to freshwater tanks. For saltwater CO2 estimation, use a dedicated reef chemistry calculator.