The Dymax Crystal CO2 Drop Indicator, crafted from premium glass, offers a straightforward and accurate way to monitor CO2 levels in planted aquariums. With the use of a pH reagent solution (sold separately), the color of the indicator changes to signal whether the CO2 levels are excessive, optimal, or insufficient. This package includes a glass drop checker and suction cup, with dimensions of 2" x 1" for the small size and 2.5" x 1.2" for the large size.