Thanks for the interest and inquiry.
We measure the concentration of CO2 in water for example by measure the head space gas.
This make use of Henrys gas law regarding concentration in air and liquid.
The measurement are made via a space cup or a gas permeable membrane.
You could do this using a simple k30 sensor;
I’m not familiar with the operation of aquarium but 30 PPM of CO2 in water sounds extremely low for water standing a state of equilibrium.