There are many colormetric methods to measure CO2.

The real question is, do they provide reliable and accurate measurement?
Are they cost effective?

A light meter that measures color and intensity will run about 150-300$ at the least and that assumes that they will make it and not charge 500-1000$.

Note, the pH meter + Kh ref solution is quite accurate and relatively cheap.

Here's some thoughts and science behind CO2 sensors:

Potentiometric CO2-Sensor

Note, IR analyzers of gas phases are very inaccurate(20ppm + or - ) in most cases. If you have lots of $$$$, then you can get much better resolution.

These would be great if they had any accuracy also:

http://www.thermo.com/eThermo/CMA/PD...tPDF_30697.pdf

But that's the rub, no accuracy.

And they cost more.

CO2 specific ISE probes work well, but they are not cheap either:

PASPORT® pH/ORP/ISE with Temperature Sensor

And

Ammonia ISE Kit

Note, there are a few different ions there, you need to add various chemicals to rule out interferences by other ions, you cannot simply use some of them continuously in the tank due to the potporuii of ions we normally have in there.

Some ISE background:

IonSelectiveElectrodes

bionmr-c1.unl.edu/421-821/Lectures/421-821-chapter-23.ppt

These guys have decent prices and good selection and are helpful/knowledgable

Ion Selective Electrodes ISE

Regards,
Tom Barr