Start by scrubbing the threads on the C02 tank with an old toothbrush to remove and dirt. This ensures that the tank connects cleanly.
To connect the regulator to the aquarium, tighten the nut with a crescent wrench. Also tighten down the regulator screw that adjusts the flow of C02 from the tank so that no air can escape.
Stand away from the regulator in case the glass on the pressure dial blows out while opening the valve. Slowly open the valve far enough to allow C02 to flow through to the aquarium. Verify that you have opened the valve enough by checking the aquarium throughout the first day of use.
Use a mixture of water and a few drops of soap to detect leaks in the connection between the regulator and the aquarium. Spray the soapy water solution on the connection and look for bubbling that indicates a leak. If you find a leak, start over from Step 1 and make sure the threads are clean and the connection between the regulator and the tank is tight.
Turn on the C02 and adjust the pressure by turning the rod-shaped handle between the gauges. Set the gas pressure to 12 to 15 psi as indicated by the pressure gauges.