Drop Checker reliability, under "ideal" measurement


Lifetime Charter Member
Lifetime Member
Mar 3, 2007

Drop checkers have become suspect in their ability to accurately reflect CO2 levels of the planted tank. As I understand it, the key issues are that CO2 levels

*normally fluctuate throughout the day
*fluctuate with variation in filter or powerhead capacity to evenly distribute CO2
*and because of circulation issues, can be different at different locations of the tank

So the problem is that a drop checker reading only reveals CO2 levels at that particular part of the tank, at that particular time of day, and with the distribution/circulation of water for that particular period of time (which will change over time as a function of filter flow, etc.).

The issue seems not to be accuracy of drop checker measurement, but stability of actual CO2 levels to which the drop checker is sensitive. Have I got that right?

So my question is - under circumstances where there is serious and equivalent distribution of current/flow throughout the tank (e.g., empty tank, multiple powerheads all possible directions run 24/7), is the drop checker reliable under these conditions?

No one would run a tank this way, but if the answer is "yes, probably, kind of", then that suggests continued utility of the drop checker, under conditions where issues of flow are carefully addressed (the latter Tom has generously repeated for the hard of hearing, many times..). My interest is just trying to get a handle on what the key problems are, and how those problems might be approached - without having to return to similar problems of pH/KH charts.


Guru Class Expert
Jan 22, 2008
Vista, Ca
I think in all reality the drop checker is completely pointless. Not only for the reasons stated above but also for the simple fact that if enough co2 is getting into the tank the plants will let you know... and if you are a few bps high, well its just going to be wasted any how with a drop checker or not... People should save their $$ and just work on getting the plants to respond by slowly adjusting co2.


Plant Guru Team
Lifetime Member
Sep 23, 2007
South Florida

The real big issue with drop checkers is that they take time to react to the c02 content in the water. The lag is anywhere from 2 hours upwards. So when the checker is viewed, you are seeing what is was X time ago, not what it may be at the viewing time.

All you really 'know' is that at some point in the past, your c02 was 'maybe' sufficient.......

Only a c02 meter that reads the dissolved c02 gas will work to give real time measurements.

They are 2K in price and thus not common.

Still a checker is useful for the novice as they learn more about c02.....

I use them every now and then just to see that the levels seem the same over time........

Henry Hatch

Guru Class Expert
Aug 31, 2006
I think that drop checkers have some value. For example I use the drop checker to determine the bottle change schedule for diy yeast. I observe when the level starts to drop and use as that as a guide to determine when to change.

Also I will move the drop checker to different locations in the tank to get some idea of how good my circulation is.

Drop checkers are flawed, but not without some value.


Lifetime Charter Member
Lifetime Member
Jan 24, 2005
Sacramento, CA
Most of the inaccuracy with drop checkers is because they rely on the equation that relates CO2 concentration to pH and KH. The equation includes raising 10 to the pH power. So, tiny errors in pH make big errors in concentration of CO2. Then, there is the inherent problem of using a color to indicate the concentration of CO2, when it is hard to discriminate between the color corresponding to 6.4 pH and 6.8 pH - a big error band.

The drop checker simply helps you to avoid being gun shy with the CO2 bubble rate, fearing that every twitch of a fish indicates CO2 poisoning. That fear kept many of us from getting more than 5-10 ppm of CO2 in the water, thinking we were way above that. Now, we can use the drop checker and know that a "twitching" fish isn't related to the CO2 when the drop checker is still blue. Treat the drop checker as an aid to learning.