TOOL 07
CO2
Calculator.
Estimate dissolved carbon dioxide from your KH and pH readings using the classic relationship. A quick way to sanity-check a planted tank before fish start gasping.
Formula: CO2 (ppm) = 3 × KH × 10(7 − pH). Assumes carbonate hardness is the dominant buffer. The drop-checker relationship is approximate — always watch your fish, not just the number.
ABOUT THIS TOOL
CO2
Calculator.
The CO2 Calculator estimates how much dissolved carbon dioxide is present in your aquarium water using the well-known relationship between carbonate hardness (KH) and pH. In a tank where carbonate is the main buffer, the two values move together in a predictable way: as CO2 increases, pH falls. Measuring KH and pH therefore gives a useful estimate of available carbon without any expensive equipment.
Carbon dioxide is the single biggest growth factor in a planted aquarium, but it is also the most dangerous addition a hobbyist can make. Too little and plants stall; too much and fish and shrimp suffocate within hours. This tool gives you a fast, practical reading so you can keep injection in the healthy 15–30 ppm window instead of guessing.
The calculation relies on a formula that assumes KH is accurate and the water is not heavily influenced by other acids such as tannins or pH buffers. Those caveats are why the result should be treated as guidance. For critical setups, a drop checker with 4 dKH reference solution remains the gold standard.
HOW TO USE
Using the
CO2 Calculator.
- Test your KHUse a liquid KH test kit and record the carbonate hardness in degrees (dKH). KH from a strip test is less accurate — a liquid drop kit is strongly preferred.
- Test your pHMeasure pH with a liquid test kit or a calibrated pen. Take the reading during the photoperiod, when CO2 is at its working concentration.
- Enter both valuesType your KH and pH into the fields above. The estimate updates instantly — no button to press.
- Read the CO2 estimateThe primary result shows dissolved CO2 in parts per million. A typical injected planted tank targets 20–30 ppm.
- Check the safety levelThe level readout tells you whether you are short, ideal, high, or dangerous. Treat red as a prompt to reduce injection immediately.
- Cross-check with a drop checkerFor anything you rely on, confirm the estimate with a drop checker in the tank. If they disagree, trust the drop checker and re-test your KH.
FAQ
CO2
Questions.
What CO2 level should a planted tank have?
Most injected planted aquariums run best between 20 and 30 ppm. Below about 15 ppm growth slows noticeably; above roughly 35 ppm fish and shrimp can become stressed, and anything approaching 40 ppm is dangerous.
Why does my result look extreme?
The formula is very sensitive to pH. A small pH error, or water buffered by something other than carbonate, can produce a wildly wrong number. Re-test with accurate kits before acting on a surprising value.
Can I use this if I do not inject CO2?
Yes — a low-tech tank will typically read single digits to the low teens depending on KH and pH. The tool is just as useful for confirming that your non-injected tank is not accumulating dangerous CO2 at night.
Does temperature affect the reading?
Temperature has a minor effect on the KH–pH–CO2 relationship but far less than pH measurement error. Focus your accuracy budget on KH and pH testing.
Is the formula the same as a drop checker?
They are based on the same principle. A drop checker is effectively a controlled KH reference in a small volume, which makes it more reliable than inputs from the wider tank.
My fish are gasping but the number looks fine — what now?
Reduce CO2 injection first and increase surface agitation. Fish behavior outranks any calculation. Gasping means insufficient oxygen or too much CO2 regardless of what the estimate says.