First, I stopped bothering about CO2 years ago. In any normal balanced aquarium, there is going to be CO2 produced by the fish and biological actions; decomposing mulm gives off CO2 (which is another reason not to vacuum the substrate in planted tanks
). I've no idea what it measures in my aquaria, but I can be very certain of one thing--there is more than enough of it.
CO2 is usually found in tap water. Many suggest when test pH of tap water that you let the water stand (in a glass or jar) for 24 hours. This is to allow the CO2 to escape at the surface, and as we all know, CO2 lowers pH, so if you do this and there is CO2 in the tap water, the pH will be higher after 24 hours. My tap water varies by .1 so there is not that much in the tap water here.
The fact that my plants are obviously thriving means I have a balance between light and nutrients in all my aquaria. As as I have mentioned before in a couple of threads, even in my spare plant tank with no fish, the plants are growing well. So, CO2 must be coming from somewhere and must be adequate.
While typing this, I see Natalie has responded, saying much the same. Don't bother with CO2, it is going to be there. Keep the light minimal and balance the minerals (via fertilizer) and you're set. [I also have a CO2 test kit, years ago, prob not working now, but I could never figure it out anyway; and as the plants continued to grow fine, I decided it was fussing over nothing.] And speaking of flourishing plants, my E. bleheri have sent out numerous flower spikes during the past couple of weeks, I have never seen so many plantlets. I must have more than 20 on 3 plants. Obviously they are happy.