take a glass of tank water and let it sit overnight. Compare the pH of the glass's water to the pH of the tank water. If the PH is 1.0 lower, the amount of CO2 is 30-40 ppm. If it is .7 lower, the amount of CO2 is 15 to 20 ppm, and if it is 1.3 lower, it is 60-80 ppm. The kH/pH test is not completely accurate so this is another method to determine what CO2 numbers are. If you are into doing pages of math, look up the kA value for H2CO3 and HCO3- and you can work out more accurate numbers. With CO2 yeilding a diprotic acid in water, the math becomes more complex but is doable. If you are not into complex math and reading, here is a the pH - pH method formula
ppmCO2=degassed ppm CO2*10exp(degassed PH-PH)
degassed ppm CO2 would range from 2-5 ppm depending on where you live. If you are sea level use 5. If you an the rocky mountains use 2 otherwise pick a number in between. Degassed pH is the pH of the water you let sit overnight.