Is The Ph Drop The Best Way To Measure Co2?


New Member
Sep 27, 2020
Here's a little write-up someone gave me:

The "1.0 pH drop" is an oversimplification you can not and should not apply without thought. Firstly, the higher the KH, the higher the buffering capacity and the exact same level of CO2 would result in a higher pH. Secondly, this idea completely ignores that there are many other influences on pH than just CO2.

Soils buffer, nitrifiers release J+, wood realses tannins, rocks dissolve, phosphate also buffers, your tap may come with extra bases in the first place, etc etc. These are just some lurking variables that make the pH drop a poor measure of co2 in your aquarium.

A pH of 6.6 and 4° dKH is the only situation where you can apply the ph-KH-CO2 table, because all that math doesn't work if you have other acids in the water too. The chart only describes water that has some KH or co2 in it. If you add another acid or base to the water, the pH and KH will change for reasons unrelated to co2.

Most people look for 30mg/l of co2 in their water. If your pH and dKH is 7°, they match green (preferable co2 level) at 21mg/l. So, according to the chart, to achieve 30mg/l in an aquarium with 7 pH, you will need to raise your dKH to 10° in order to have that optimal 30mg/l co2. You can also use the formula [CO2_ppm = 3* KH * 10^(7-pH)] to figure it out.

This is why a drop checker is always the preferable method of measuring CO2. Because CO2 is the only pH altering susbtances that notably moves through air, the drop checher essentially filters your data and by looking at the pH change in water that is surely only influences by CO2 (besides a known KH), you get a proper reading of the CO2 level in your tank.

What do you all think?