co2 indicator question?


Guru Class Expert
Jan 22, 2008
Vista, Ca
I purchased the red sea co2 checker and in the directions it mentions that it will not give correct readings if the water KH is below 4, how do i know without a test kit if i have 4 kh water. If i dont how can i go about getting it above 4?

The plants are pearling and the tester is still blue.



Prolific Poster
May 23, 2007
Unfortunately, those directions make things simpler than they really are. For your drop checker to work properly you need water of a known KH with no other buffers. That means distilled or deionized water with a known quantity of baking soda added. Most people use a 4 dKH solution so that the drop checker is green when the CO2 concentration is about 30 ppm.

You'll need to either make or buy this 4 dKH solution. There are directions on this board and elsewhere on the internet for how to make it. It's not that hard, but you'll either need a KH test kit or a fairly sensitive scale.


Lifetime Charter Member
Lifetime Member
Nov 21, 2007
Los Angeles, CA
It definitely doesn't take the drop checker 24 hours to start working. You might want to double check something. Take your drop checker out and clean out the bell portion of it on the inside -- you can leave the 4dKH solution in it. You may notice a scum (protein?) layer across the bell. If your drop checker stays green after your CO2 goes off (it will even stay green all nite), you can rest assured that you have the scum layer. For some reason, this layer causes the CO2 to keep building up -- there is no longer an exchange of CO2 with the water.