The pH/KH formula
In water where carbonate hardness is the main buffer, CO2, pH and KH are linked by a simple relationship:
CO2 (ppm) ≈ 3 × KH (dKH) × 10(7 − pH)
1 dKH equals about 17.85 ppm (mg/L) of CaCO3, so the calculator converts if your test kit reports ppm.
Why the estimate can be wrong
The formula assumes carbonate is the only thing affecting pH. Anything else that buffers or acidifies the water (driftwood tannins, peat, active substrates like aquasoil, phosphate buffers, pH-adjusting products) pushes the reading away from the truth, often making CO2 look much higher than it is. Many planted-tank keepers therefore prefer the pH drop method: let a sample of tank water fully degas overnight, measure its pH, and compare it with the tank's pH when CO2 is running. A drop of about 1.0 pH unit is commonly used as a guide for roughly 30 ppm.
Target levels
Many high-tech planted tanks aim for around 20–30 ppm CO2. Raise CO2 gradually and watch your fish; gasping at the surface is a sign to reduce it.
Frequently asked questions
How do I calculate CO2 from pH and KH?
Use CO2 ≈ 3 × KH × 10^(7 − pH), with KH in dKH. For example, KH 4 and pH 6.8 give about 19 ppm.
What pH should I aim for with CO2 injection?
It depends on your KH. The calculator shows the pH that matches your target CO2; with KH 4 and a 30 ppm target it is about 6.6.
Is a drop checker more accurate?
A drop checker with 4 dKH reference solution avoids interference from other buffers in the tank water, but it reacts slowly (an hour or more). Many keepers use it together with the pH drop method.