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.

Aquarium CO2 / pH / KH Calculator

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.

Example, limits, and tips

At KH 4 dKH and pH 6.6, CO2 = 3 × 4 × 10^(7 − 6.6) = 12 × 10^0.4 ≈ 30 ppm — right at the upper target. The formula assumes carbonic acid is the only thing moving pH, so peat, tannins, or phosphate buffers will inflate the reading; in those tanks trust a drop checker over the number. Aim for 20 to 30 ppm during the photoperiod, ramp CO2 up before lights on, and watch fish for gasping at the surface as an early sign of overdosing.