By injecting pressurized carbon dioxide at a slow, steady rate into the aquarium we are dissolving carbon dioxide into the water column that your fish and plants reside in. First, we need to understand how to measure aquarium CO2 levels. This is commonly done one of two ways;
Visually. Using an aquarium CO2 indicator and reagent liquid that reacts with the amount of CO2 contained in it, we can see the liquid in the bulb turn from blue (insufficient carbon dioxide) to green (sufficient carbon dioxide) to yellow (too much carbon dioxide). The reagent will turn green when carbon dioxide saturation reaches approximately 30ppm.
By studying the GH/KH/PH relationship When CO2 Gas is dissolved into the water, some of it turns into carbonic acid and increasing acidity. CO2 levels can thus measured by comparing the pH of the water column before injecting CO2 and during CO2 injection. Acidity will increase slowly as carbon dioxide saturation levels are built up to the point of the carbon dioxide off gassing from the water column.
By counting bubbles per second. This is the least accurate way, but can be used along with an aquarium CO2 indicator to provide accurate results.
In order to measure and record these levels for your own records and knowledge you will need to record two numbers - the pH of the tank before using CO2, and the pH of the tank after carbon dioxide has reached saturation point. Saturation times depend on bubbles per second of carbon dioxide being injected, tank volume, flow rates, and other variables such as if you are running a lid or not can also effect your saturation.
The general rule of thumb is to aim for a 1 point PH drop to be in the sweet spot of 30PPM. The tank will have 30+ppm of CO2 where the KH is between 1 to 10 degrees KH. Low KH can result in poor plant growth because low KH water does not allow enough carbon dioxide to saturate the water. Instead of retaining 30 PPM, the water will off gas CO2 at a rate that does depend on the hardness of your water.