Setting CO2 with the pH-drop method

The idea

The KH/pH chart needs one assumption that rarely holds in planted tanks: that carbonate is the only buffer. The pH-drop method avoids it by comparing the tank water with itself. Take a sample of tank water and let it stand or aerate it until its excess CO2 has escaped to the air. It keeps all the tank’s other buffers, so the only difference between the degassed sample and the tank with CO2 running is the CO2.

The arithmetic

At a fixed alkalinity, the carbonate equilibrium gives CO2 proportional to 10^(pK1 − pH) ^[1]. pK1 is the same for both samples at the same temperature, so the ratio of CO2 between the tank and the degassed sample depends only on the pH difference:

CO2 in tank ÷ CO2 in degassed sample = 10^(pH drop)

pH drop	CO2 relative to the degassed sample
0.5	3.2 ×
1.0	10 ×
1.3	20 ×
1.5	32 ×
2.0	100 ×

The method tells you a ratio. To turn it into mg/L you need the CO2 content of the degassed sample.

What is the baseline?

Water left in contact with air settles at the CO2 level set by Henry’s law: the concentration is proportional to the CO2 pressure in the air above it. Using the NIST WebBook’s Henry’s law constant of about 0.034 mol/(kg·bar) at 25 °C ^[2] and the NOAA Mauna Loa monthly mean of 426.07 ppm for September 2026 ^[3], water at equilibrium with outdoor air holds about 0.65 mg/L of CO2 at 25 °C, a little more in cooler water. The equilibrium guide shows the calculation. Tropica gives the natural CO2 level of an aquarium as 0–0.5 mg/L ^[4], close to the same figure.

Multiplying through:

pH drop	CO2 if the sample is at 0.65 mg/L	if the sample still holds 2 mg/L
1.0	6.5 mg/L	20 mg/L
1.3	13 mg/L	40 mg/L
1.5	21 mg/L	63 mg/L

The spread shows the weakness: an incompletely degassed sample can make the same pH drop mean three times as much CO2. Tropica notes that without injection, plants rely on the CO2 released by fish and bacteria ^[5], so tank water is not simply at air equilibrium, and a short degassing may not bring the sample all the way down. The equilibrium value also depends on the air the sample is in contact with; the NOAA figure is the outdoor background.

Doing it

1. Fill a clean glass with tank water and leave it to stand, or aerate it with an air stone, long enough to reach a stable pH. Measure that pH.
2. Measure the tank’s pH at the point in the day when CO2 should be at its target, usually a few hours after injection starts.
3. Subtract and read the ratio from the table.

Use the same pH meter or test for both readings and at the same temperature; the method relies on a difference, so a constant offset in the meter cancels out, but reading resolution does not.

Limits

• Baseline uncertainty, as above, is the largest source of error.
• Resolution. A 0.1 pH reading error in either sample changes the ratio by about 21–26 %, depending on its direction.
• Timing. CO2 in the tank rises after injection starts and falls through the day as plants use it and gas escapes at the surface.

For a reading that does not depend on a baseline at all, combine the pH drop with a drop checker, or, in water with no other buffers, use the KH and pH calculator directly.