How to set up a CO2 system

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. Plants use more carbon than all other nutrients combined, so having ample access to carbon dioxide has a huge impact on growth, health and coloration of aquatic plants.

In the picture below 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 cylinder does need to be refilled every few months.

1. CO2 Cylinder

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. Larger tanks have economies of scale when purchasing CO2 refills. Having much larger cylinders (20lb+) drives CO2 cost down tremendously.

2. Regulator (with Solenoid)

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 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. 

This entire system is usually sold together as a set. There is not much assembly required by the end user except for attaching on the Regulator unit onto the cylinder.

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

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. This often comes in a set with other accessories. 

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. May come as a set with the regulator.

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

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 valves - 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 already come with built-in check valves.

Steps to install a new CO2 system

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 (May require 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 screw threads 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 needle valve knob turned on until the regulator receives power. If the regulator did not come with a solenoid, gas will be emitted when the needle valve 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/Inline reactor/Inline atomizer.

4. Turn on the power if the Regulator is attached to a solenoid. If all the valves are turn on you should see CO2 gas being released in the bubble counter.

After 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.

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 of the tank water before CO2 injection starts. This is the baseline pH 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 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.

Click on here to go to How to measure CO2 levels accurately

Timing - When to turn CO2 on/off

We would use an automatic timer to begin CO2 injection at least 2-3 hours before the tank lights come on. For users that inject at lower injection rates, it can take many hours for water to get saturated with CO2. 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.

General FAQs

Does my planted aquarium need a strong light to benefit from CO2 injection?

No. Low light planted aquariums also benefit greatly from CO2 injection. Low light levels coupled with CO2 injection is one of the most stable, easy to manage combinations out there. Some tank styles are especially suited for this combination, such as lower light nature style and Iwagumi tanks. The usage of CO2 allows carpets to grow in using low light levels, which keeps algae at at bay.

How long does a CO2 cylinder last before it needs refilling?

A 3 liter CO2 cylinder holds between 1.5 to 1.8kg of CO2 gas. Higher injection CO2 rates will deplete the cylinder much more quickly than low injection rates, so how long a cylinder lasts vary quite a bit. As a very rough gauge this can supply a 60 litre aquarium with CO2 for about 2 to 4 months. 

What size of cylinder should I get?

The main reason to get larger cylinders is that CO2 costs per kg fall steeply as cylinder size increases. The cost of CO2 refill is usually calculated as cost of labour (fixed price) plus cost of CO2 (charged at per kg). The majority of cost is in the price labour, the per unit cost of CO2 is actually very low. The downside of using larger cylinders is the difficulty of transport. We recommend using the largest cylinder possible that fits your cabinet that you can transport yourself. Some gas companies offer transport options for larger cylinders.

Switching up to a huge cylinder from a tiny one can drop monthly CO2 costs by a factor of 10 times.

Can I use airline tubing or do I need CO2 specific tuning?

Some diffusers with ultra fine membranes for producing fine CO2 mist require higher working pressure from the regulator to work. CO2 specific tubing is required in such an application. CO2 specific tubes are generally stiffer/thicker and more lasting and they also give tighter connections that prevent gas leaks. 

In simple low pressure diffuser systems, airline tubing can be used but should be checked regularly for leaks and wear condition.

How do I prevent gassing my fish from injecting too much CO2?

Having a system where there is good gaseous exchange is the best prevention against injecting too much CO2. This can be done through a combination of having a clean water surface and good turnover between surface tank water and deeper layers of water.

Click here to read more on achieving good gaseous exchange.

Should I use an air stone when I am injecting CO2?

You could, but using an air stone is not an efficient way to manage gaseous exchange in a CO2 injected aquarium. Click here to read more on achieving good gaseous exchange without using an air stone.

Air stones are rarely used in planted aquascapes. Good surface turnover is the main mechanism used to ensure good oxygen levels.

What bubble rate should I start with?

Some regulators allow multiple manifolds to be connected to supply multiple tanks with CO2 from a single cylinder.

Bubble counters can have different bubble sizes which can add quite a bit of variance to this estimation. A good starting point is about 1 bubble per second every 50 litres of tank water. For tanks aiming for high CO2 saturation rates, the bubble rate could be 4 to 5 times that rate. Ultimately CO2 levels should be tuned using the pH drop method as described on this page.

However, click here to read more on achieving good gaseous exchange if you are aiming for higher CO2 saturation levels.

Can I add fish when I have CO2?

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. Fish can also be added after a water change, when CO2 levels are not elevated.

Can I have shrimp with CO2 ?

Yes, shrimps are very common in CO2 injected planted aquariums. Shrimps have a high tolerance for elevated CO2 levels.

Here are some helpful links:

Click on here to go to How to measure CO2 levels accurately

Click on here to go to Plus the limits of CO2 injection safely

Click on here to go to Why inject CO2? Isn't it unnatural?