The CO2, pH and KH Relationship in Planted Tanks

Dissolved CO2 forms carbonic acid and lowers pH, while KH buffers it. Learn how the pH drop reveals CO2, how the drop checker works, and the 1-pH-drop rule.

In a CO2-injected planted tank, three parameters move together: dissolved CO2, pH and KH. Understanding how they interact is the basis for estimating CO2 without an expensive sensor, and for using a drop checker safely. The chemistry is straightforward once you separate cause from effect.

How CO2 lowers pH

When carbon dioxide dissolves in water it forms a small amount of carbonic acid, a mild acid that lowers the pH. Crucially, adding CO2 lowers pH but does not change the water's alkalinity (KH). So in a tank where CO2 is the only thing changing the acidity, the size of the pH drop is a direct readout of how much CO2 has dissolved.

What KH does

Carbonate hardness (KH) is the carbonate and bicarbonate content of the water; one degree KH (dKH) equals about 17.85 mg/L as calcium carbonate. KH is the buffer that resists pH change. Higher KH means the water holds its pH more firmly, so more CO2 is needed to push the pH down by the same amount; soft, low-KH water shifts pH with much less CO2. This is why the same CO2 dose produces a bigger pH swing in soft water than in hard water.

The 1-pH-drop rule

A practical target is a relative pH drop of about 1.0 unit, measured from before CO2 is switched on to the stable point after it has built up. That roughly corresponds to around 30 ppm of CO2 (some put a full 1-point drop nearer 35 ppm), a level that is both sufficient for plants and generally safe for livestock. Because it is a relative drop, the rule works at any starting KH.

The drop checker

A drop checker turns the same chemistry into a colour. It is a small reservoir of pH indicator solution made up with reference water of a known KH (commonly 4 dKH), separated from the tank water by an air gap. CO2 diffuses from the tank water into the indicator and shifts its colour: blue means too little CO2, green is the target (around 30 ppm), and yellow warns of too much. Because CO2 must diffuse across the air gap, the drop checker lags real-time conditions by roughly two hours.