Drop checkers are used to monitor the CO2 levels in the aquarium: the pH indicator solution inside the bulb section will change color according to the amount of CO2 dissolved in the aquarium water.
The drop checker is the best tool to monitor CO2 levels continuously. With some practice and experience it can be a precise tool to measure the amount of CO2 dissolved.
These drop checkers are U-shaped and must be placed over the edge of rimless tanks with the bulb on the outside of the tank. They fit up to 3/4 inch thick glass.
IMPORTANT: You will also need a Drop Checker Reagent (sold separately) to use this item.