CO2 cylinders come in various sizes. CO2 tanks come with a fitting known as a CGA320 fitting, which is standard in North America. Europe and Asia use different industrial standards. The general best piece of advice is to get the largest tank that you can afford and/or is feasible for the space that you have.
The next essential piece of equipment required for a pressurized CO2 setup is the regulator. A regulator takes the tank pressure of the CO2 tank (normally at ~850 PSI or more, depending on the ambient temperature) and reduces it to a lower pressure. Individuals normally look for a regulator with two gauges. This means there are two pressure dials. The first pressure dial (high pressure dial) will indicate the pressure in the CO2 tank (i.e. the amount of CO2 that is remaining in the tank). The second pressure dial (low pressure dial, also known as the delivery pressure), will be the pressure that the regulator is bringing the CO2 down to. This is usually set anywhere from 5-20 PSI, depending on the size of your tank, and the desired bubble rate.
Sometimes, the term dual stage used. Note that dual stage and dual gauge are not the same. These terms are sometimes used interchangeably, but this is incorrect. Dual stage refers to an additional body within the regulator that allows the pressure to be dropped in two stages, hence the name. Wheher you purchase a single or dual stage regulator is up to you; dual stage regulators are the "premium" regulators, and will work reliably for our purposes. Single stage regulators will also work well for our purposes, and are often cheaper than dual stage regulators.
A needle valve is a piece of equipment that takes the delivery pressure of the regulator and further drops the pressure down to the very fine flow rate that we require for aquarium purposes ( Flow rates as are often referred to as "bubbles per second"). A metering valve is the "high end" needle valve.
Needle valves work by restricting the flow of gas via a small needle (hence the name) that can be opened/closed via a screw/caliper handle. In general, higher quality needle valves/metering valves will have allow finer control by having more threads. This means that it takes more turns of the handle to change the flow of CO2, meaning you get finer resolution (i.e. if you turn a needle valve 1 turn and get an increase from 1 bubble per second to 10 bubbles per second, you would have a hard time adjusting your flow. However, if you turn another needle valve 1 turn and only get an increase from 1 bubble to 2 bubbles per second, you can achieve much finer control). A good quality needle/metering valve is essential.
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).
A bubble counter allows us to easily determine the flow rate of the CO2 that is going into the aquarium. Flow rate is often refered to as "bubbles per second." The bubble counter is filled with fluid (it can be water, glycerin, or even mineral oil. The latter two are sometimes preferred because bubbles flow through the liquid slower, making it easier to count. In addition, they do not evaporate like water does), and as gas flows through, bubbles are generated so that you can count your bubble rate.
Placing a check valve is important to prevent a back siphon from occurring. If water were to back siphon, it could go back through the needle valve, destroying your regulator diaphragm. To protect your investment, a check valve provides good protection for a few dollars.
Airline tubing, you would not be able to get the CO2 from your CO2 tank into your aquarium, so it goes without saying that you will require airline tubing.
A pH controller (such as those made by Milwaukee) will allow you to determine the pH of your aquarium water on a continual basis. By hooking up the pH controller to a solenoid (which is part of your CO2 system), you can have your pH controller inject CO2 when your pH goes above a certain set point, and have it stop injecting CO2 when your pH drops below a certain set point.
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