Help - need some more details on CO2 Indicator and how to buy...

OK, so I've been reading lots lately on how CO2 drop checkers are the best way to figure out CO2 concentrations.

I've also read that the Dennerle CO2 Indicator is one of the better ones out there and that it comes with a reference water standard while most others do not.

How do I go about getting one of these test kits? I've only found it on-line at one place for $20. (although everyone seems to have the refill for it....) Is it really that much better than the Red Sea or Azoo test kits and worth the price?

How does the test kit work? Is it one that attaches inside the aquarium (like the Red Sea indicator) or is it external where you fill up a vial and get an individual reading...

Thanks for helping a rookie out.


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Lifetime Member
Jan 24, 2005
Sacramento, CA
All of these CO2 indicators work the same way. They are a method for measuring the pH of a sample of water with a known KH, and which is otherwise pure water. That makes the pH/KH/ppm CO2 charts accurate. All of them must be placed in the aquarium.

The ones you can buy all use an air gap to separate the known KH water sample from the aquarium, and the air gap makes the two batches of water reach an equilibrium where both have the same ppm of CO2 in them - which is why this works.

Using 4 dKH distilled or DI water as the water sample means that at 30 ppm of CO2 the color of the sample will be green, because bromothymol blue indicator, used in low range pH test kits, is green at pH of 6.6, and a KH of 4 with a pH of 6.6 equals 30 ppm of CO2.

You can DIY an indicator using gas permeable membrane to replace the air gap and get a faster reaction time, if you keep the ratio of membrane area to fluid volume as large as possible. A disc of fluid 1/16" thick will have a response time of about 20-30 minutes to reach about 2/3 of its final reading, compared to the air gap type, which reaches the same point in about an hour. If you fill the membrane type fluid volume with a piece of sponge, to reduce the volume of fluid, keeping the same area of membrane, you can reduce the response time to around 5 minutes or so.

The Dennerle unit seems to be out of stock everywhere, for some reason.