CO₂ calculator (pH & kH)

Dissolved CO₂ in a freshwater tank is estimated from two readings: CO₂ ≈ 3 × kH × 10^(7 − pH), with kH in dKH. At pH 6.8 and kH 4 that works out to 19 ppm — inside the 15–30 ppm range most injected planted tanks are run at.

The formula

The 10^(7 − pH) part is exponential, which is why small pH movements mean large CO₂ movements: every 0.1 of pH is roughly a 26% change in the estimate. It is also why pH alone tells you nothing — you need kH.

Reference table

Estimated CO₂ in ppm for common freshwater readings. Green is the usual target range, amber is worth watching, red is a problem.

How to use the number

• 15–30 ppm is where most CO₂-injected planted tanks sit. Below 10 ppm, growth stalls and slow growers lose the competition to algae.
• Above 40 ppm livestock start gasping at the surface, especially shrimp and small rasboras. Back the injection off and add surface agitation.
• Measure at the same time each day. pH rises through the photoperiod as plants strip CO₂, so an evening reading always looks "lower CO₂" than a morning one.
• Calibrate the inputs. A pH pen that is 0.2 off moves the estimate by roughly 60%; kH from a strip is a guess at best.
• Cross-check. A drop checker is a second opinion, not a replacement for the maths — but if the two disagree wildly, trust neither until you re-test pH.

What breaks the estimate

• Marine water. Borate alkalinity invalidates the carbonate assumption entirely.
• pH adjusters and buffers. pH-minus products, alkaline buffers and some substrates move pH without changing CO₂.
• Active aqua soil. Fresh soil strips kH and pulls pH down; the reading looks like high CO₂ when it is mostly the substrate.
• Organic acids and tannins. They lower pH without adding CO₂.
• A single reading. One number on one afternoon is a data point. Trends over a week tell you what the tank is actually doing.

Questions

What is the CO₂ formula from pH and kH?

CO₂ (ppm) ≈ 3 × kH × 10^(7 − pH), with kH in dKH. Every 0.1 of pH is roughly a 26% change in the result.

What is a good CO₂ level?

About 15–30 ppm for an injected planted tank; 30 ppm is the usual target. Under 10 ppm plants stall, over 40 ppm fish and shrimp suffer.

Why do two tanks at pH 6.8 read differently?

kH. At kH 4 you get 19 ppm; at kH 10 the same pH means 48 ppm.

Does this work for saltwater?

No — marine alkalinity is not mostly carbonate, so the formula does not apply.

Why does my reading change between morning and evening?

Plants consume CO₂ in the light and respire at night, so pH swings daily. Measure at the same time of day and look at the trend.