Jan!
There are basic things you need for a system, and then there are more advanced things you may have if you desire.
First you need a regulator. You need a needlevalve on the regulator to ajust the flow going into the line. You need a bubble counter inline to visually see how much gas is going thru the line. You need a device to dispense gas into your aquarium. Think very seriously about a reactor as opposed to a diffusor. A reactor mixes the gas with water. This is the most efficient and well worth the money. A diffusor is a glorified airstone, and just wastes gas, and is a waste of money. This is what you will find in those ultra cheap EBay systems. Your friend Vicki used reactors: the power reactors and the Reactor 1000. She was very fond of them.
The extra things include a solenoid valve, which is an electronicaly controlled shut off valve, and a pH controller. If you plug the solenoid into a timer with your lights, it will shut off the C02 when the lights go out. Or you can plug the sollenoid into a pH controller. The pH controller has a probe in the aquarium that reads the pH. When the pH reaches the level you have set, it signals the solenoid to shut off. When the pH begins to rise, it signals the solenoid to turn the gas on. This is the ultimate automation, but it is not hassle free. You need to re calibrate the probe every so often.
So the most critical things are the regulator, needle valve, check valve, bubble counter, and the reactor. If you need more help, just give me a call.
