Drop Checkers/CO2 Indicators-Why and How


Junior Poster
Jul 17, 2010
I agree that the drop checker method is far from accurate. I use it mostly as an alerting method when the co2 is in the low range. If it becomes dark green/blue I know I have a problem...

It still gives an indication that something happens when the color changes.

I am currently trying to find a balance and playing with a few versions of reactors. The drop checker gives my clues but I don't rely on it as a method to quatify the amount of co2 in the tank. More as a visual indicator that I am on the right track.
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Lifetime Charter Member
Lifetime Member
Jan 5, 2009
Washington, DC
billypete, this thread explains what Tom was referring to and offers a very clever way around the problem.
Wet;47924 said:
For example, this is a regular drop checker with 4dKH.


If it is perfectly green, I have 30ppm CO2. But I am human and a nerd and my eyes suck. If my eyes can only tell shades of green +/- 0.2 degrees pH, it only says I have between 18.9 and 47.5ppm CO2.

Pretty crappy, right?

So, here's a second drop checker at 9dKH. It is green, so it says I have between 42.9 and 107.8ppm CO2.


I use both of them together, and both are green, so now I know I have between 42.9 and 47.5 ppm CO2.

The calculator:
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Junior Poster
Mar 11, 2009
Excellent idea provided you can gaurentee the 9 dkh solution ? So does This mean your ph should be 6.6 +/- .1 ?? I have a ph controller ... if I'm injecting co2 via this set to 6.6 this should mean I have 30 ppm co2 ? Or should I rely on the drop checker method and use the controller more as a visual gauge that things are stable ..I.e. run the co2 all day on a solenoid night shut off switch and use controller as a constantly readable ph meter ?
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Lifetime Members
Lifetime Member
Aug 25, 2006
Hey billypete. The calculator tug linked will also give you instructions on how to make your own dKH solution, and will be as accurate as the purity of the baking soda (NaHCO3) and the accuracy of your tools and scale. If you want a 9dKH reference, this will get you there or really, really close.

The problem with your numbers below relying on the pH meter is that there's too many buffers in our tanks to accurately derive CO2.

The problem with one drop checker is that it is slow and our measurement of the reading (color perception) sucks.

The problem with two drop checkers is it is slow and can only work off a (pretty safe) assumption on the crappiness of the above.

The problem with CO2 flow meters are they are expensive.

But it is important to understand that all of these are just indicators and the most important measurement is plant health. This is no different than any other test, be it a spectrometer, LaMotte, AP, or whatevs for the other nutrients.

So anyway, my advice is to notice that most guys with nice plants use these tools to get ballparks or watch trends occasionally. The rest of the time they're tending to and watching their gardens. In regards to your last question, I'd suggest you use both tools in hand to see where the tank is, but in the meantime let your plant and fish health, not numbers, dictate how you move with the tank. Then use your tools to get a new ballpark. Don't rely on the tools: plants are the best measurement.


Prolific Poster
Oct 4, 2005
Livingston, Scotland
Cute idea.

(BTW Wet, you've spelt Litre wrong on your web site in your form. :)

I've often wondered why manufacturers don't bring out a pH meter with a CO2 button.

If a pH probe is immersed in a 4dKH solution the pH meter can be used to calculate the level of CO2. It just needs a different algorithm to display the CO2 level or a chart for pH users.

Then you'd have a accurate digital read out!