As you may know, carbon plays a vital role in the healthy growth of your aquarium plants. Next to lighting and fertilizers, it is crucial to provide an adequate amount of carbon. The most viable form of carbon, the easiest form for plants to use, is carbon dioxide gas. In nature, this occurs naturally as flowing water exchanges gasses on the surface, making carbon available to submerged plants. In the aquarium, surface agitation is enough to promote some gas exchange, but not enough to provide adequate levels of carbon. There are other options for adding carbon as well, such as some commercially available liquid forms, but the active ingredients in these solutions can be harmful and have been anecdotally known to harm live plants and shrimp. In addition, these products are far less efficient and far more expensive over time than the CO2 setup I'm going to show you here.
The first thing you will need is a 5lb aluminum CO2 cylinder and a trusted source to refill it. I simply googled CO2 tank in Google Maps and found four places within 10 miles of my house that sold and refilled CO2 tanks. They were welding and gas supply stores. I called each of them and chose the one with the best price, $87 for the tank, $9 for the CO2 gas.
While CO2 gas is not inherently dangerous, any pressurized container can be, so be sure to purchase from a respected and trusted source. Avoid any used cylinders that have been sitting around in a garage from private sellers, unless your willing to get it inspected first.
Gas cylinders have a manufacturers date and are in need of inspection every 5 years. When you refill your tank every few months, they will exchange the cylinder with a fresh one, so you will most likely never need to worry about getting your cylinder inspected. If you do, the store you bought it from or filled it at should be able to handle it.
Next you'll need a decent regulator. For the beginner setup, you won't need anything more complicated than a single stage, dual gauge, regulator sized for 5lb tanks with a solenoid and bubble counter. A solenoid is simple electrically powered valve that when turned off, shuts off the supply of CO2 gas from the cylinder to the aquarium. This allows you to plug in the solenoid and aquarium light into a timer and the CO2 will turn on and shut off automatically with your light. You will never have to constantly adjust the regulator valve every morning and night. The bubble counter is how you measure the amount of CO2 going into your system. 1-2 bubbles per second is standard. The stage refers to how many CO2 lines you can attach to the regulator (dual stage for multiple tanks). The gauge refers to the needle gauge which measures the pressure inside the tank. Dual gauges measure the pressure in the cylinder and in the regulator. Green Leaf Aquatics, CO2Art, and Fzone have regulators as described above for between $100 and $200.
Then, you will need a CO2 diffuser. These work much like air stones, but have much finer holes in the ceramic so the size of the bubbles of CO2 entering the aquarium are as small as possible. There are multiple sizes and styles of diffusers. Choose one you like between 13 and 27 mm depending on the size of your tank. Be sure to attach the diffuser near your return or close to a powerhead so the bubbles are forced throw-out the aquarium.
Inline diffusers are also an option. They attach inline to your return hose from your canister filter. They are more efficient for the following reasons. (1) The bubbles are ejected from the return and spread throughout the aquarium more efficiently, (2) the diffuser is not exposed to light which keeps it cleaner, and (3) the diffuser is usually much larger, allowing for more even gas diffusion so larger bubbles don't form.
Finally, you will need a length of airline tubing (vinyl or silicone), a check valve, drop checker, and smart strip outlet. I highly recommend the smart strip outlet for multiple reasons. First, you can control it from your phone or voice with google assistant and Amazon Alexa. Second, you can plug your light and CO2 into the same surge protected, grounded strip, but control them separately with as many schedules as you desire. The CO2 should come on 30 min to one hour after the light and turn off one hour before the light goes off. Remember, when the solenoid plugged into the outlet looses power, it shuts the CO2 gas supply off. Lastly, the smart strip outlets are inexpensive. I got mine at my local home improvement box store for $30 and the app was free.
The drop checker is a device used to measure the amount of CO2 in your tank. They are easy to install and only cost around $10.
With the cylinder, regulator, tubing, check valve, diffuser, drop checker, and outlet, your total comes to around $275. I saved quite a bit by getting my regulator 2nd hand and shopping around for the cylinder.