Cal Aqua Labs’ “Double Check” drop checker is the only all-glass dual-compartment CO2 checker that allows the user to accurately compare the CO2 level in your aquarium side-by-side against a standard reference. The top chamber holds the CO2 indicator solution, whereas the bottom chamber carries a CO2 reference solution. The reference solution color corresponds to 30 ppm of CO2 – a level that is widely accepted for good plant growth in most aquariums. No more guessing on the shades of green. No more looking back-and-forth at color sheets to compare….