Drop Checker sits inside of the aquarium and measures the amount of Co2 saturation in the water column. Add pH solution to the drop checker, then place inside the aquarium and the Drop Checker will begin to measure the Co2 levels in the water. It is recommended to place Drop Checker on the opposite end of the aquarium from where your Co2 Diffuser is. For example, if your diffuser is on the right pane of glass in the aquarium, place Drop Checker on the left pane of glass. This way, you do not get an excessive Co2 dosage reading from being too close to the diffuser.