A bubble counter is a very simple device that helps you measure the injection rate of CO₂. It is usually constructed of a glass or a plastic tube, and two pipes - one long and another short. The long pipe is where the gas goes into the tube, and the short one is where the gas goes outside of the tube (and into the aquarium). If the tube is partially filled with water, one can see the gas bubbles coming out of the long pipe, and determine the amount of gas he adds to the aquarium (usually measured in bubbles per second).