A CO2 system for the planted aquarium has 3 main parts and includes a number of useful accessories. This is one area where it is better to spend on quality parts, than to solve issues arising from using cheaper quality parts later on (problems such as leaking tanks or inaccurate tuning of CO2 injection rates). Being cheap on this aspect can costs more in the long run.
Refer to picture 1. This is the metal container that stores Carbon dioxide in compressed form. It comes in various sizes. For a 40gallon aquarium tank, a 5lb cylinder lasts for around 3-4 months at a good injection rate. Cost of CO2 refill for a 40 gallon tanks should cost less than $5 USD a month.
Refer to picture 2. This device releases CO2 gas in a controlled manner from the cylinder. The solenoid connects to a power outlet; the regulator releases CO2 when power is supplied to the solenoid. By itself, the solenoid is simply an 'on/off' valve that turns on when the power supply is turned on. The flow rate of the CO2 gas released is controlled by a needle valve.
There are typically 2 pressure gauges. One shows the internal pressure in the cylinder (the one with the higher PSI markings) and the other shows the output pressure. The internal pressure tells how full or empty the cylinder is.
Refer to picture 3. From the regulator, we would typically connect the outflow tubing to a bubble counter, which would be used to estimate the rate of flow of CO2 into the tank. The tube that connects to a diffusion device, of which there are 2 pathways. It can be an in-tank diffuser/atomizer where the CO2 tubing from regulator feeds to device inside the tank itself, or an external inline-diffuser/atomizer or CO2 reactor where the CO2 tubing from the regulator feeds into device that attaches to the filter outflow tube/ pipe and follows the flow of water from the filter/pump back into the tank.
I recommend using an inline atomizer or reactor (more efficient but also far more complicated to tune correctly). Both are installed along the filter outflow line which reduces the number of items in the tank itself (this is preferred by aquascapers, as it makes for better visuals: less intrusive background equipment parts).
1. CO2 cylinder - Pressurized CO2 gas is stored inside, this can be bought from gas suppliers, pet shops or re-purposed from paint ball / fire-extinguishers.
2. CO2 Regulator with attached solenoid - the regulator (may come with one or two gauges) down regulates the pressure from the cylinder and allows you to inject CO2 at tiny amounts from a highly pressured cylinder. The solenoid is a small device that usually comes already attached to the regulator - it allows you to time the injection of CO2 using a power timer plugged into the wall power outlet.
3. Bubble counter - some models can be attached directly to the regulator. This allows you to observe CO2 gas exiting the regulator.
4. CO2 tubing - this tubing is easily cut and connects the flow of CO2 between parts. CO2 resistant tubing is harder, but lasts a long time.
5. CO2 diffuser/Inline atomizer/reactor - this is the exit point for your CO2, and each one introduces CO2 into the tank in a slightly different way.
6. One way check valve - This valve can be connected between the bubble counter and the regulator to prevent tank water from back-flowing into the regulator when CO2 is turned off. Some bubble counters come with in-build check valves.
If you are installing your CO2 system for the first time (or for a new tank), I suggest the following steps:
1. Attach regulator to cylinder, tighten using a spanner. Make sure that there is a functional O ring securing the connection between the regular and the cylinder. No white tape/thread seal/plumber's tape should be used in the connection as the seal is formed between the O ring, cylinder and regulator and not by the threading on the regulator/cylinder.
2. Open valve on top of cylinder. You should see the pressure gauge for cylinder pressure jump. If you have a solenoid attached to your regulator, no gas will be emitted even if the regular knob is turned on until the regulator receives power. If the regulator did not come with a solenoid, gas will be emitted if the knob on the regulator is turned on. If you hear sounds of gas escaping even when the regulator knob is turned off, chances are the connection between the regulator and cylinder is not tight enough. Close the valve on the top of the cylinder and tighten the connection.
3. Attach bubble counter and run the CO2 line to your diffusion device.
4. After attaching the regulator to the cylinder (you would usually need a spanner for this step) and setting up all necessary tubing, I would check for leaks using soapy water; using a paintbrush, paint soapy water over all joints in the system. If there is a leak, large soap bubbles will be visible.
Checking for leaks is often neglected, but it is akin to failing to check the handbrakes on a new car: there is low probability of a real problem, but it can become a big problem if discovered at a later stage.
5. Connect up the power and check that CO2 flows through the system when power is turned on.
Next I would take the pH/KH of the tank water before CO2 injection starts. This is the baseline pH/KH value.
Getting this baseline value is important, as we will want to target a one point drop in pH with the injected CO2.
As CO2 injection starts, the pH of the tank water will drop as a portion of the injected CO2 is converted to carbonic acid.
With the risk of oversimplification, a good starting target would be attaining CO2 in the region of 20ppm -35ppm. If using a bubble counter - start at one bubble per second per 20 gallons of tank volume, then slowly tune upwards while measuring the pH drop. (It can take a few hours for the pH to drop fully and stabilize). This can be a process that takes quite long to play out if you want an accurate gauge of CO2 levels.
The way to gauge this is through measuring the change in pH/KH as a proxy for the amount of CO2 dissolved, as described in more detail here. This does require the ability to test pH and KH which is a necessary investment for newer aquarists. Generally, if your KH is between 1 to 10, aiming for a 1 point drop in pH (from when CO2 injection is off to peak saturation) will put you at good levels.
If livestock is already present while CO2 is being adjusted, livestock must be observed closely for signs of excessive CO2 saturation. There can be a significant lag effect between CO2 adjustment and the time it takes for water to be saturated.
I would use an automatic timer to begin CO2 injection at least an hour, if not 2 hours before the tank lights turn on. Plants draw CO2 most strongly in the initial hours of light exposure, so having good CO2 levels upon lights on is ideal.
One can turn CO2 injection off an hour before the lights off. This does depend on how short the light cycle is; if the light cycle is 6 hours or shorter, I would recommend running CO2 for the entire light period.
For my tanks, lights and CO2 injection are turned on for a duration of 8 hours per day.
For beginners afraid of algae, using a short light cycle such as 6 hours can be helpful. For stable, matured tanks, light duration can be pushed to 10hours+ without adverse effects.
When releasing livestock into the tank, always release them during the period that CO2 injection is turned off. When levels build up gradually during the next CO2 cycle they will have some time to adapt.
Here are some helpful related links:
1. section for PH / KH explained