Tool
CO₂ calculator
Estimate dissolved CO₂ from pH and KH — the standard check for a planted tank.
Your result
About 19 ppm CO₂
Ideal range for a CO₂-injected planted tank.
What this calculator does
In water, CO₂ forms a weak acid, so injecting CO₂ lowers pH — and how far depends on the water's carbonate hardness (KH), which buffers that change. Because the relationship is predictable, you can estimate CO₂ from a pH and KH reading. The tool uses the standard aquarium formula:
CO₂ (ppm) = 3 × KH × 10(7 − pH)
When to use it
- Setting a target for a CO₂-injected planted tank.
- Checking that you are in a safe range for fish.
- Understanding why the same CO₂ changes pH differently in soft and hard water.
How to use it
- Measure pH with a reliable kit or meter, ideally mid-photoperiod when CO₂ is at its peak.
- Measure KH with a test kit.
- Enter both and compare the result with the interpretation below.
How to read the result
| CO₂ (ppm) | Meaning |
|---|---|
| Under 10 | Low. Fine for non-injected tanks; plants grow slowly. |
| 15–30 | Typical target for a CO₂-injected planted tank. |
| 30–40 | High. Watch fish closely for stress. |
| Over 40 | Too high. Increase surface movement and reduce injection. |
Worked example
A tank at pH 6.8 with KH 4 gives about 19 ppm CO₂ — a sensible level for a planted tank. Drop the pH to 6.4 at the same KH and the estimate jumps to roughly 76 ppm, well into the danger zone for fish.
Limitations
- It is an estimate. Other acids (tannins, aquasoil, pH buffers) also move pH and will make the reading wrong.
- Bad test kits mislead. pH inaccuracies change the estimate a lot, because the formula is exponential.
- Watch the fish first. Gasping at the surface is a more important signal than any number.
Frequently asked questions
Why does soft water show more CO₂?
Because KH is the buffer. With low KH, the same CO₂ drops pH further, so the formula reads a higher CO₂ for a given pH.
What is a safe CO₂ level for fish?
Most guidance treats roughly 15–30 ppm as the working range and above 30–40 ppm as risky, but individual fish differ. Always watch behaviour.
Does this work without injected CO₂?
It estimates whatever CO₂ is present, from respiration and the atmosphere. In a non-injected tank it is usually low.
Related guides
Sources and research notes
The CO₂–pH–KH relationship is a standard water-chemistry result widely used in planted-tank keeping; the estimate assumes carbonate buffering is the main influence on pH.