Here is where I'll share my experiences with setting up pressurized CO2 for my aquariums. I was quite nervous going into pressurized CO2. I would read horror stories and conflicting tips online. Over the past couple years with some trial and error, I currently have 2 tanks that are running with CO2 with another 2 set ups coming up soon, all with 5 lb CO2 tanks (the tanks contain 5 lbs of CO2 gas). Hopefully I will be able to help those that are on the fence or confused about how to get started.
Overall Tip
Don't go cheap! You don't need to get top of the line parts, but you definitely don't want to go the cheapest route. Quality matters and will cost you less over time. With that, the upfront cost will be a little high, but cost over time, if done right, shouldn't be that high.
Parts
CO2 Tank
The CO2 tanks are fairly simple. The important part is there are 2 main types of tanks that will determine what type of regulator to get: paintball tank or a bigger normal tank. For the purposes of this article and due to my own experiences, I will be referring to a normal tank. The smallest of these standard tanks is a 5 lb tank, meaning 5 lbs of CO2. The next one up that some may use is a 10 lb tank. Generally slightly more expensive, but twice as much for just a little big increase in physical space.
Speaking of price, in general in my area, there are no actual refills of tanks, just replacements. So don't go around looking for a specific tank and material. Just go to a place (like Robert's Oxygen in my area) and purchase a full tank. On your next visit when your tank is empty, bring in your empty tank and exchange it for a full one. In my area, the price of a new full tank is about $130 with an exchange running at just under $30.
That may seem like a lot of money, but for reference, 5 lb of CO2 running on a 10-20 gallon tank should last you at least 6 months. So after the initial purchase, that is hopefully at most $60 a year on CO2, or $5/month.
Regulator
I think one of the most important parts to not skimp on is the regulator. This is the device that connects to the CO2 tank and transitions the pressurized CO2 into a more manageable pressure. My first regulator was a cheap Chinese one, which seemed to have small leaks in it. This ended up costing me more money in the long run due to having to get more CO2 refills more often that I should need to.
Things to look for in a regulator:
Dual Stage - Supposedly prevents "End of tank dump". Not sure how true it is, but better safe than sorry
Solenoid - a device that connects the regulator to an outlet. If it is not plugged in, no gas will be going through. Allows you to hook up your regulator to an outlet/wall timer. Otherwise you will be manually turning on/off your gas at morning/night
Needle nose valve - Should come with most regulators. Allows you to fine tune the amount of gas coming through
My personal recommended regulators:
GLA PRO-1 - GLA is "Top of the line" and very expensive. Comes with a built in bubble counter
Premium Aquatek Regulator - My favorite budget friendly one. Comes with an inline bubble counter. Cheaper on Ebay direct from Aquatek than on Amazon.
Diffuser
This is the device that helps dissolve the CO2 into the water. There are many different types of diffusers: inline, bubble (different types there), passive. I find the easiest way that is also very effective is the bubble diffusion. This is where the CO2 gets forced through a medium, typically a ceramic disk and comes out into your tank as a very fine mist of CO2 bubbles.
I don't have a particular brand I would recommend over others since there are so many, but I would suggest looking at reviews. The ones I currently use are glass tube diffusers and an atomizer.
Bubble Counter
The bubble counter is actually optional but recommended. This helps you get an idea of how much and how fast CO2 is coming out of your regulator. Some regulators come with a bubble counter attached to it.
Tubing
You can use either CO2 specific tubing or general airline tubing. Supposedly general airline tubing will leak out a little CO2 as well as get hard and deteriorate over time. I have one set up using CO2 specific tubing and others using airline tubing.
My recommendation is if you have a lot of airline tubing around, use that. If you don't, maybe try the CO2 specific tubing. I don't think there is a significant amount of CO2 loss through normal air tubing and it is quite cheap to replace when it gets old.
Extras
These are recommended extra items to help you with your CO2
Big Ol' Wrench
If you don't have one yet, you will want a big wrench to screw in the regulator into the CO2 tank. You will not be able to screw it in by hand. If you have a cheap Chinese regulator, it may come with its own special wrench.
CO2 Drop Checker
To know if you have enough or too much CO2, get a CO2 drop checker and liquid. I would recommend getting one that you think looks good as it will sit in your tank (or some will partially be in your tank). For simplicity, I would also suggest purchasing the drop checker liquid as well instead of trying to make your own. Make sure to not mix in or add water into your drop checker liquid. If done correctly, I think the liquid in the drop checker should last a long time. Over time once you have your settings dialed in, you may not really need to look at it much.
Check Valve
When your CO2 is off, over time it will dissolve into the water, causing a water to suction back up your tubing. To prevent the water going all the way back to your regulator, add in a check valve. Some bubble counters will come with a check valve built in.
Electrical Timers
Assuming your regulator has a solenoid, you will want to get electrical timers to turn on and off your CO2.
Putting Things Together
Once you have your items here are some overall steps on putting your things together
Screw your regulator onto your CO2 tank with your big wrench
Your CO2 tank should come with a washer. Unless you are using something like a Brass Permaseal from GLA, you will need to put the washer between where your regulator and CO2 tank meet. This should get crushed a bit to fill in any gaps between the 2 pieces of metal to prevent any gas leaking out. You will need to put in a bit of muscle to do this!
Attach your tubing. This means attaching your bubble counter too (if you are using one) and your check valve. Don't forget to put water in it
Place your diffuser in your tank - A good location is away from your filter input, and maybe under your filter return. The flow will help disperse the CO2 around your tank.
Open your CO2 valve on your CO2 tank. Doesn't need to be way open, just a turn or so. You should see one of the gauges on your regulator go up.
Turn your main knob on your regulator. This should set the pressure on your secondary stage. If using a normal diffuser, about 20-30 psi should be good. If using an atomizer, it may need 35-40 psi.
Turn your needle valve (the knob closest to the airline tube on your regulator) to off.
Plug in your solenoid, you should hear a click from it indicating it is turned on.
Slowly open the needle valve a little bit, you will see a lot of bubbles in your bubble counter
You may need to wait for 10 minutes or so for things to settle down. Your bubble counter should settle down and you should start seeing bubbles come out of your diffuser
If after 10 minutes the bubbles are coming out really slowly, you can open the needle valve more to increase the amount of CO2 coming out
Monitoring and Tips
Best tool to monitor is the drop checker. Keep in mind the color changes take up to a couple hours to change so start of with small slower changes. Good location to place the checker is opposite side of the tank from where your diffuser is
Over time the drop checker will start losing air - it is just getting dissolved into the water. I take a dropper full of air and just replace the air inside
The bubble counter over time will most likely start to lose water. There is nothing to worry about as long as you are fine with your CO2 output. You can refill it on your next CO2 tank refill/replacement.
Flow will help disperse the CO2 around your tank better. If you don't have any flow, you may find yourself using a little more CO2
I run with an airstone. Yes, I am losing some CO2 due to this offgassing, but I run my tanks with fish and shrimp and I find it better safe than sorry. Keep in mind that CO2 and Oxygen are not mutually exclusive. Having more CO2 does not mean less Oxygen, but you want to make sure you have Oxygen!
With this hopefully it makes things a little easier for you and pressurized CO2. I went through a lot of worry and trial and error beforehand.