What is the CO2 from pH and KH formula?

Dissolved CO2 in ppm is approximately 3 times KH (in dKH) times 10 to the power of (7 minus pH). This comes from the carbonate buffer equilibrium, where adding CO2 forms carbonic acid that lowers pH for a given carbonate hardness. The constant 3 folds in the unit conversions for aquarium use.

What CO2 level should a planted tank target?

Most high-light planted tanks run 20 to 30 ppm dissolved CO2. Below about 15 ppm plant growth is carbon-limited; above 30 ppm fish and shrimp can suffocate. The 30 ppm point corresponds to roughly a 1.0 drop in pH from the fully degassed value at a given KH.

Why can this reading be misleading?

The formula assumes CO2 is the only acid affecting pH. Tanks dosed with phosphate buffers, peat, tannins, or other acids will read a falsely high CO2 because something other than carbonic acid is lowering pH. Treat the number as an estimate and watch livestock behaviour.

How do I raise or lower dissolved CO2?

Increase the CO2 injection rate (bubbles per second) or improve circulation to dissolve more gas, which lowers pH and raises the calculated CO2. To reduce CO2, cut injection or add surface agitation to off-gas it. Adjust slowly and re-test, since fish respond to sudden CO2 swings.

Does temperature affect this calculation?

Temperature shifts the carbonate equilibrium constants slightly, but for the 22 to 28C range typical of planted tanks the standard 3 times KH times 10^(7 minus pH) formula is accurate enough for setting injection rates. Use a drop checker as a cross-check.

What is the Aquarium CO2 / pH / KH Calculator?

Uses the carbonate equilibrium relationship to compute dissolved CO2 in ppm from your aquarium pH and KH readings, with a planted-tank target band. For planted tank hobbyists tuning CO2 injection. It runs free in your browser on Gera Tools, with nothing uploaded.

Aquarium CO2 / pH / KH Calculator

Name: Aquarium CO2 / pH / KH Calculator
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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In a planted aquarium, dissolved CO2 is the single biggest lever on plant growth, yet you cannot measure it directly. This calculator infers it from two things you can measure — pH and carbonate hardness — using the carbonate buffer equilibrium, and tells you whether you are in the healthy 20 to 30 ppm band.

How it works

Carbon dioxide dissolves to form carbonic acid, which lowers pH for a given carbonate hardness. The standard aquarium relationship is:

CO2 (ppm) = 3 × KH(dKH) × 10^(7 − pH)

A lower pH at the same KH means more dissolved CO2. Every 1.0 unit drop in pH roughly multiplies CO2 tenfold, which is why a tank degassing overnight drifts its pH upward.

Worked examples

Well-tuned high-light planted tank: KH = 4 dKH, pH = 6.6 CO2 = 3 × 4 × 10^(7 − 6.6) = 12 × 10^0.4 ≈ 30 ppm — right at the upper target. A good starting point for demanding plants.

Moderate planted tank: KH = 5 dKH, pH = 6.8 CO2 = 3 × 5 × 10^(7 − 6.8) = 15 × 10^0.2 ≈ 24 ppm — solidly in the target zone, comfortable for most fish.

Insufficient CO2 (plant growth slow): KH = 5 dKH, pH = 7.2 CO2 = 3 × 5 × 10^(7 − 7.2) = 15 × 10^(−0.2) ≈ 9.5 ppm — below the 15 ppm minimum for meaningful plant growth. Increase injection rate.

Risk zone: KH = 3 dKH, pH = 6.4 CO2 = 3 × 3 × 10^(7 − 6.4) = 9 × 10^0.6 ≈ 36 ppm — above 30 ppm. Watch for fish gasping at the surface; reduce CO2 injection.

The pH-KH-CO2 relationship in practice

The formula describes a triangular relationship: fix any two values and the third is determined. This has several practical implications:

KH acts as a buffer. A higher KH makes pH more resistant to the acidifying effect of CO2 injection. A tank at KH 8 will have a smaller pH drop from the same amount of CO2 injection than a tank at KH 2, because the carbonate ions neutralise the carbonic acid. Tanks with very low KH can experience sudden pH crashes if CO2 is added too quickly.

Day/night CO2 swings. Plants consume CO2 during the photoperiod, which would raise pH. Your injection system replaces it, keeping pH stable. At lights-off, injection stops but plants switch to respiration (producing CO2), and then overnight the CO2 degasses — pH typically rises 0.3 to 0.5 units by morning in a well-planted tank. This daily swing is normal.

Testing timing matters. Test pH in the afternoon, several hours into the photoperiod after CO2 has been running, not first thing in the morning when the tank has been degassing all night. Morning readings will show artificially high pH and suggest insufficient CO2 even if injection is working correctly.

When the formula is unreliable

The formula assumes carbonic acid is the only source of acidity. It breaks down when:

Peat, driftwood, or blackwater extracts add humic and tannic acids that lower pH independently of CO2. The calculated CO2 will be inflated.
Phosphate buffers or pH-down chemicals artificially depress pH without reflecting dissolved gas.
Very high or very low KH (below 2 dKH or above 15 dKH) pushes the calculation outside the range where the simplified formula is accurate.

In soft, blackwater tanks, use a drop checker with a reference solution as your primary CO2 indicator; treat this calculator as a secondary cross-check.