It is designed to reduce pressure surges, but also makes a good dust filter. It is installed between the tank and the regulator. Next you need a way to control the flow, because you can't just send 50 psi of CO 2 into an aquarium. CO 2 dissolves very readily in water, you only need a tiny trickle - about a bubble or 2 per second. There are two common approaches at this point - manual control or electronic control of the gas delivery.
Needle valves have a tiny hole with a small needle that screws into that hole.
The gas comes through that hole and is restricted by the needle. The needle has a very fine turning mechanism that lets you make extremely precise adjustments in the output flow. The valve I use has about 50 turns from off to full on, and I can turn it one revolution to get from one bubble out per second to two bubbles out per second. With such a fine control of the gas flow, you can experimentally find an equilibrium point where the CO 2 is dissolved at the same rate it is utilized by the tank and released into the atmosphere. With electronic control, you put a pH probe into the tank that measures the pH of the water and produces a control signal that can be used to shut off the gas flow via an electronic air solenoid valve.