Maybe this will help
The major componants are the regulator, which is the gauges that go on your C02 tank, a needle valve on the regulator which is what you use to ajust how much gas is going into your aquarium, (most regulators made for aquariums come with a built in needle valve, but if you buy a cheap regulator like what is used for a beer tap, you would need to add a needlevalve), a bubble counter which is filled with water and is either attached directly to the regulator or attached to the tubing running from the regulator to the aquarium, and finaly a device to either mix the gas with water, (a reactor) or a device that bubbles the gas into the water, (a diffusor).
Reactors are more efficient because the gas is totally absorbed into the water and the enriched water is pumped into your aquarium. Diffusors are less efficient because no matter how fine the bubbles are, many simply rise to the water surface and disapate. With diffusors it is sometimes difficult to reach the higher levels of C02 in your water.
There are both internal and external reactors. An internal reactor sits inside your aquarium and is usually attached to a small pump. External reactors can either sit under your tank or hang on the back of the tank and can be connected to a cannister filter or pump.
There are two other componants that are more optional. First is a Solenoid valve or sometimes called a magnetic valve. This attaches to a regulator and is an electronic shutoff valve. It plugs into an electrical outlet. When it is plugged in, the C02 flows, when it is unplugged or the power is shut off, it closes the C02 shutting it off. A solenoid is used in one of two ways: plugged into a timer with your lights so that the C02 is shut off at night, or plugged into a pH controller.
A pH controller gives you the ultimate automation. A probe inside the aquarium reads the pH. When enough C02 is added to your aquarium to bring the pH down to what you have it set for, the controller signals the solenoid valve to shut off. As the plants use the C02 and the pH begins to rise, it signals the solenoid to turn on keeping the C02 at a more constant level with much less pH swing.
There are currently two brands of regulators that include a needle valve, bubblecounter and solenoid valve. JBJ and Milwaukee. JBJ also has a check valve which Milwaukee does not. A check valve will prevent back pressure that could draw water from your aquarium into your regulator, which sometimes happens if you shut off the pump on the reactor. The other difference between these to regulators is that the JBJ has a fixed working pressure, which means when you open up the C02 valve on your C02 tank, the regulator automaticaly ajusts to the right working pressure. With the Milwaukee you have to manually ajust to the desired PSI pressure.
There are other aquarium C02 regulators that do not include the solenoid or the bubblecounter such as Aqua Medic and Ultralife. When using these you would have to buy a separate solenoid if you want to use one. They either connect inline via the tubing, or fitted onto the regulator. Some people simply leave their C02 running 24/7 without a solenoid or pH controller and live with the pH swing at night.
Some online dealers or petstores will try and sell you a C02 tank, or include it in the "system". Buying a C02 tank with your system is a waste of money in my opinion. You have to bring the tank somewhere to be filled. 90% of these places will not actually fill your tank, but instead swap it for one of theirs that is already filled. Many of these places will not even accept a tank for swap that is not one of their own. Instead you can buy a filled tank from them for 70 to $80 and refill swaps for around 10 to $15. Look for welding gas suppliers in your phone book. Every major city or area usually has several to choose from. Some people report that fire extinguisher suppliers will refill tanks.
You may also hear some talk on the internet about different grades of C02 gas. It is all hogwash. Any C02 gas that you can purchase is basicaly the same and suitable for aquarium use.
