"Optional" (?) Parts
Here now, are the optional parts of a pressurized CO2 setup. While the aforementioned CO2 tank, regulator and needle valve are absolutely essential, the following pieces of equipment, while totally optional, are highly recommended.
5) Bubble Counter
7) Drop Checker
8) Miscellaneous equipment (check valve, airline tubing)
9) "Luxury" items (pH controller)
Let us begin with 4) Solenoid
A solenoid is an electronically controlled valve that opens/closes depending on whether electricity is flowing through it or not. For pressurized CO2 purposes, we normally use a "normally closed" solenoid. This means that when there is no electricity, the solenoid is closed, and no CO2 flows. When there is electricity, the solenoid is open, and CO2 flows.
A solenoid provides the option of putting your CO2 onto a timer and/or a pH controller, so that your CO2 will turn on/off automatically. This is beneficial, as it can prolong the amount of time your CO2 will last (i.e. rather than having it on for 24 hours, you can turn it off at night, when plants are no longer photosynthesizing).
Here are some good solenoid brands that I can recommend (in no particular order):
Burkert (Model #: 6011)*
Clippard (Model #: MME-2PDS-D110)*
Fabco (Model #: 3853-04-A287)*
Of these 3, the Clippard is the only one (as far as I know) that has the handy feature of having a red LED that indicates when the solenoid is open, and CO2 is flowing. However, many users have reported that the solenoid does get quite warm (sometimes to the point where you cannot leave your hand on it comfortably). The other three brands do not seem to have this overheating issue, but also do not possess the same LED.
Most solenoids run ~$20-30, but you can easily find a cheaper solenoid on eBay (see below and/or PM me for the actual link...)
*Note: These model numbers are for solenoids with a 1/8" female NPT ports on either side. If you wish to have different port sizes, I can forward you the PDF with the appropriate information.