CO2 when dissolved in water releases carbonic acid (H2CO3) thereby raising the acidity of the aquarium water. The pH scale ranges from acid (-7) to alkaline (+7), meaning that a greater level of acidity will lower the pH in your aquarium.
With most aquarium plants liking a pH of between 6.5 and 7.8 and your fish needing oxygen to breathe rather than carbon dioxide, you really need to optimize the levels of CO2 injected into your water column.
Optimizing CO2 injection helps to stabilize your pH and dissolved CO2 levels which means happier fish, plants and much less chance of the appearance of the dreaded black beard algae.
Checking the level of dissolved CO2 in your tank is relatively easy. You need to use a CO2 drop indictor kit.
Easy to use. Blue means CO2 is too low, Yellow means too much CO2 and green is just right. Cheap and effective although a little cumbersome and labor intensive as a method of level control. If you are really looking at optimizing your CO2 and stabilizing your pH you need to look at a pH controller.
A pH controller digitally measures the pH in your aquarium and will open and close the attached solenoid to your CO2 system. You can set the parameters as you wish according to your own pH needs.