In planted aquariums aquarists often utilise CO2 as a means of keeping plants healthy. As such it is important to keep an eye on the CO2 saturation as if it goes too high it can easily do harm to your fish and even kill them.
A simple way to get a visual indication of the CO2 saturation is with a CO2 Drop Checker. This is typically a small bulb that "samples" the water CO2 Saturation and contains some PH Indicator agent mixed with 4DKH Solution. Importantly the visual indicator is slow to react meaning the reading is from some hours prior to when you are looking at the colour of the liquid in the bulb.
Simply water saturated with CO2 will lower PH. If the liquid in the bulb is Green then the CO2 levels in the aquarium are ok if yellow too high and if blue too low.