CO2 System 101 - for beginners

A CO2 system injects carbon dioxide into the water of the aquarium. This allows aquatic plants to have greater access to carbon dioxide, allowing a wider variety of aquatic plants to grow well in the tank.

In the picture above you can see the CO2 system in its entirety connected to the filter.

A CO2 system sounds complicated, but it is actually quite simple once you get the parts connected. CO2 systems do not require much maintenance over time, although it may require some tuning when first set up. However, the tank does need to be refilled every few months.

3 Core parts

A planted aquarium CO2 system has 3 main parts and a number of useful accessories. This is one area where it is better to spend money on quality parts than to have to fix problems later caused by using cheaper quality parts (problems such as leaking tanks or inaccurate tuning of CO2 injection rates). Being cheap in this area may cost more in the long run.

CO2 Cylinder

See Figure 1. This is the metal container that stores carbon dioxide in compressed form. It comes in several sizes. For a 40 gallon aquarium tank, a 5lb cylinder will last about 3-4 months at a good injection rate. The cost of refilling CO2 for a 40 gallon tank should be less than $5 USD per month.

Regulator (with Solenoid)

See Figure 2. This device releases CO2 gas from the cylinder in a controlled manner. The solenoid plugs into an electrical outlet; the regulator releases CO2 when power is applied to the solenoid. The solenoid itself is simply an on/off valve that turns on when power is applied. The flow rate of the released CO2 gas 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 indicates how full or empty the cylinder is.

Diffusion Device (In tank diffusers, inline-diffusers, reactors)

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.

​We 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). 

Equipment checklist:

1. CO2 cylinder - pressurized CO2 gas is stored inside, this can be purchased from gas suppliers, pet stores or repurposed 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 in tiny amounts from a high pressure 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 a wall outlet.

3. Bubble counter - some models can be attached directly to the regulator. This allows you to monitor the CO2 gas coming out of the regulator.

4. CO2 hose - this hose is easily cut and connects the flow of CO2 between parts. CO2 resistant tubing is harder, but lasts longer.

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 flowing back into the regulator when the CO2 is turned off. Some bubble counters come with built-in check valves.

If you are installing your CO2 system for the first time (or for a new tank), we suggest the following steps:

1. Attach the regulator to the cylinder and tighten with a wrench. Make sure there is a working O-ring securing the connection between the regulator and the cylinder. No white tape/thread seal/plumber's tape should be used in the joint as the seal is formed between the O-ring, cylinder and regulator and not by the thread on the regulator/cylinder.

2. Open the valve at the top of the cylinder. You should see the cylinder pressure gauge jump. If you have a solenoid attached to your regulator, no gas will flow even with the regular knob turned on until the regulator receives power. If the regulator did not come with a solenoid, gas will be emitted when the knob on the regulator is turned on. If you hear the sound of gas escaping even when the regulator knob is turned off, the connection between the regulator and the cylinder may not be tight enough. Close the valve at the top of the cylinder and tighten the connection.

3. Attach the bubble counter and run the CO2 line to your diffuser.

4. After attaching the regulator to the cylinder (you will usually need a wrench for this step) and installing all the necessary tubing, it is a good idea to check for leaks with soapy water: using a paintbrush, apply soapy water to all the joints in the system. If there is a leak, large bubbles will be visible.

Checking for leaks is often neglected, but it is like not checking the handbrakes on a new car: the probability of a real problem is low, but it can become a big problem if discovered later.

5. Turn on the power and verify that CO2 flows through the system when the power is turned on.

CO2 systems are usually kept hidden in the cabinet together with the filter system.

How much CO2 to inject?

Next we 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.

A good starting point would be to achieve CO2 in the 25ppm - 35ppm range. If using a bubble counter - start with one bubble per second per 20 gallons of tank volume, then slowly adjust up while measuring the pH drop. (It may take a few hours for the pH to fully drop and stabilize). This can be a lengthy process if you want an accurate reading of CO2 levels.

The way to measure this is to measure the change in pH/KH as a proxy for the amount of dissolved CO2, as described in more detail here. This requires the ability to test pH and KH, which is a necessary investment for newer aquarists. In general, if your KH is between 1 and 10, aiming for a 1 point drop in pH(from when CO2 injection is turned off to peak saturation) will put you at good levels.

If animals are already present when CO2 is adjusted, the animals must be closely monitored for signs of excessive CO2 saturation. There can be a significant lag time between CO2 adjustment and the time it takes for the water to become saturated.

Timing - When to turn CO2 on/off

We would use an automatic timer to begin CO2 injection at least an hour, if not 2 hours, before the tank lights come on. Plants consume the most CO2 in the first few hours of light, so it is ideal to have good CO2 levels when the lights come on.

You can turn off the CO2 injection an hour before turning off the lights. This depends on how short the light cycle is; if the light cycle is 6 hours or less, we would recommend running CO2 for the entire light period.

Most of our tanks run lights and CO2 for 8 hours per day.

For beginners who are afraid of algae, a shorter light cycle such as 6 hours may be helpful. For stable, mature tanks, the light duration can be increased to 10 hours+ without adverse effects.

Concerns about livestock?

When releasing animals into the tank, always release them during the period when CO2 injection is turned off. When levels gradually build up during the next CO2 cycle, they will have some time to adjust.

Here are some helpful links:

Section for PH / KH explained