Quick Answer — How Aquarium CO2 Is Calculated from pH and KH
Dissolved CO2 in aquarium water can be calculated from two simple test-kit readings using the standard carbonate equilibrium formula: CO_2 (ppm) = 3 × KH (dKH) × 10^((7 - pH)). This works because dissolved CO2 forms carbonic acid in water, which lowers pH in proportion to the water's carbonate hardness (KH) — so measuring both lets you back-calculate the CO2 concentration without a dedicated CO2 test kit or probe.
The widely cited target for a planted aquarium is 20-30 ppm of dissolved CO2, the range where most aquarium plants photosynthesize efficiently without stressing fish. Below 15 ppm, plants tend to grow slowly and algae can outcompete them for available nutrients; above 40 ppm, fish may show visible signs of distress such as gasping at the water's surface, since CO2 above that level starts to interfere with their ability to take in oxygen.
For a tank at pH 6.8 and 4 dKH: 3 × 4 × 10^((7-6.8)) = 12 × 10^(0.2) = 12 × 1.585 ≈ 19.0 ppm — just below the 20-30 ppm target, matching the value found on standard published pH/KH/CO2 reference tables for the same inputs.