How to Inject CO2 into your Aquarium to take your Aquascape to the Next Level 

This comprehensive guide to CO2 for your aquarium includes step-by-step instructions for installing both in-tank and inline CO2 systems, along with crucial tips, equipment suggestions, and troubleshooting strategies suitable for beginners and seasoned aquarists alike.

Why use CO2 in an Aquarium?

CO2 can instil fear in many aquarists; it sounds intimidating, technical, and expensive! We often receive inquiries from customers who are nervous about using CO2 and whether they should make the investment. Ultimately, it depends on what you want from your aquarium. If you're looking to avoid limitations on the types of plants you can grow, achieve fast results, and promote optimal plant growth, then CO2 can be very beneficial. Using CO2 can greatly enhance the overall health of your aquarium plants, which in turn supports the well-being of the animals that inhabit it.

How Does CO2 Help Plants in Your Planted Tank?

(A science-y bit)

Carbon dioxide (CO2) is essential for aquatic plants for several reasons related to their growth, photosynthesis, and overall health.

Like terrestrial plants, aquatic plants use CO2 during photosynthesis to produce glucose and oxygen. They absorb CO2 directly from the water, where it is dissolved (by the CO2 System) allowing them to convert light energy into chemical energy, which is vital for their growth. During this process, aquatic plants release oxygen as a byproduct, which is crucial for the survival of fish and other marine organisms, including beneficial bacteria. 

CO2 is a carbon source for all plants, allowing them to incorporate it into organic compounds that form the foundation of their biomass. Since plants are primarily composed of carbon, CO2 not only supports their structure but also plays a vital role in the formation of essential components such as carbohydrates, sugars, starches and more. These compounds provide strength to the stems, leaves, and cell walls of the plants.

How to set CO2 up on your Aquarium?

So now we have some basic understanding of how CO2 is essential to plants, and how it helps when added to your aquarium.... how DO you add it to your planted tank? This is where you might hear the term CO2 Injection or CO2 System

A CO2 System consists of a few key components:

1. CO2 Gas Bottle

There are various types of CO2 bottles you can use, from trade gas, (we offer a swap bottle service) to certified aluminium canisters from brands such as ISTA. 

ISTA offers two types of bottles: Face up and Face side. 

• Face up - means that the regulator is mounted to the opening located on the top of the bottle
• Face side - means that the regulator is mounted to an opening that is positioned at a 90-degree angle to the top of the bottle, allowing it to sit on the side

This is purely for space reasons and makes no difference to the way the CO2 is injected.

Why we do not recommend fire extinguishers for CO2 Systems

We advise against using fire extinguishers for CO2 systems due to their design. Fire extinguishers have a dip tube that extends throughout the length of the canister and are meant for one-time use, quickly discharging CO2 to put out flames. When the horn is activated, the pressure pushes out everything inside, including liquid CO2 and small debris, which can enter your regulator and potentially cause damage and freeze the internals due to intense pressure.

2. Regulator with Solenoid

What is a CO2 Regulator?

A CO2 regulator reduces the high pressure from the CO2 bottle to a lower, manageable level suitable for its intended use while allowing precise control of the gas flow rate to ensure the right amount is delivered to the aquarium, this is done using the dials.

What is a Solenoid?

The solenoid enables you to turn the regulator on and off. By plugging it into a smart plug, you can control the time period during which you inject CO2 into your aquarium. Without a solenoid, you cannot turn off the CO2, which can be hazardous at night since plants do not utilise CO2 at that time; instead, they absorb oxygen. Additionally, leaving the CO2 running at night results in unnecessary waste.

Solenoids are not always included with some regulators but we highly recommend getting one with it such as CO2 Art or Life Aqua Regulators which include a solenoids and bubble counters. 

Shop CO2 Regulators with built in Solenoid & Bubble Counter

3. Bubble Counter

These often come attached to the regulator, but some brands don't include them and have to be installed separately and are placed between the CO2 tubing and the diffuser.

The bubble counter provides an indication of how many bubbles per second are being injected into your aquarium. This is a simple method for measuring CO2 levels. A good starting point is to aim for 1 bubble per second for every 30 litres of water. However, this is just a rough guideline, and you should monitor your CO2 levels throughout the day, especially during the initial setup. It is advisable to install the bubble counter when you have time to observe its performance for a full day.

4. Drop Checker & Solution

How do you know when your CO2 is optimal in your aquarium? This is done with a drop checker, these come in many different shapes and designs but ultimately it is a glass or acrylic holder for drop checker solution, this pH-sensitive indicator solution reacts to the CO2 in the water and turns 3 different colours depending on CO2 levels:

• Yellow (pH around 6.0 or lower): Approximately 30-50 ppm CO2 - Dangerous for aquatic life
• Green (pH around 6.6): Approximately 15-30 ppm CO2 - Considered an optimal range for many planted tanks
• Blue (pH around 7.0 or higher): Below 15 ppm CO2 - Sub-optimal CO2 levels

The pH levels are a rough guidline, it is not uncommon for a tank to drop just below 6 during the day, most livestock can tolerate this, but the best thing is to keep an eye on your fish and ensure they aren't gasping at the surface, the more CO2 theres is in the water, the less room there is for oxygen, it is essentially a balacing act and that is why a drop checker is a really helpful tool. 

Shop Orb CO2 Drop Checker

5. CO2 Diffuser 

A CO2 diffuser is a ceramic disk with a tube on one end that connects to the CO2 tubing. It is used to disperse the CO2 by forcing it through the ceramic disk, creating microbubbles.

Inline or In-tank Diffusers

There are two main types of diffusers: 

• In-tank diffusers: placed directly inside the aquarium and release fine bubbles that rise through the water
• In-line diffusers: Connected to the 'out' line of a canister filter, injecting CO2 into the water flow before it re-enters the tank

In-tank diffusers may require more frequent cleaning and can be visible within the aquarium.

In-line diffusers tend to be more efficient at dissolving CO2 and offer a cleaner look since they are hidden outside the tank. However, they do produce a fine mist which can be made more obvious when you have a black background on your tank which can be quite distracting if you are trying to view your tank while the CO2 is running. 

The choice between the two depends on personal preference and specific tank needs.

6. CO2 Resistant Tubing

This tubing runs from the bubble counter on the regulator, into a non-return valve, and then from the non-return valve into the diffuser.

Shop CO2 Resistant Tubing

7. Non-Return Valve

This crucial device prevents water from being back-siphoned into the regulator, protecting it from damage. It should be installed outside the tank, a few centimeters before the tubing enters the aquarium.

Shop Non-return Valve

Depending on the type of diffuser you're using, you might also need a U-Bend. Typically made of glass, acrylic, or stainless steel, this component is placed on the edge of the tank and provides a smooth, seamless way to guide the CO2 tubing into the aquarium and toward the diffuser.

Setting Up CO2 with an In-Tank CO2 Diffuser

Attach the Regulator
Begin by attaching the regulator to the CO2 cylinder. Align the threads carefully and tighten it securely, you will need a wrench for this but do not over tighten. 

Some regulators come with a plastic washer that should be placed between the gas opening and the regulator thread.

Connect the Tubing
Connect one end of the tubing to the outlet of the regulator. This outlet may be on the bubble counter. Then, connect the other end to the diffuser. You will need to cut the CO2 tubing between these 2 points once you are happy with the length

Install the Check Valve
Install a check valve between the two points of tubing you just cut (before the diffuser). This will prevent water from siphoning back into the regulator.

Set Up the Diffuser
Position the diffuser in the aquarium, ideally on the opposite side from the outlet, so that the CO2 can circulate throughout the tank effectively. Ensure that the diffuser is submerged and capable of creating bubbles.

Open the CO2 Bottle Valve
Slowly open the valve on the CO2 cylinder to allow CO2 to flow.

Set the working pressure (PSI) on the regulator to 30PSI or 0.3, depending on the gauge metrics. If you are using an inline diffuser, you may need to set it to 35PSI.

Adjust CO2 Settings & Bubble Rate
Use the regulator's needle valve to adjust the bubble rate for your desired flow. Start with a slow rate, around 1-2 bubbles per second, and then adjust based on how your drop checker and livestock respond.

Observe and Adjust
Monitor the response of your plants and adjust the CO2 levels as needed. 

It’s wise to keep a backup CO2 tank, especially if you depend on CO2 for your plants' health.

Installing an Inline CO2 Diffuser

Cut your Filter Pipework
Cut the outflow pipe from your canister filter approximately 10 cm (4 inches) upstream from the outflow nozzle. Ensure the cut is straight and clean to prevent leaks

Position the Diffuser
Take the inline CO2 diffuser and place it between the two cut ends of the pipe. Ensure that the flow direction, often indicated by arrows on the diffuser, aligns with the water flow towards the aquarium.

Push the hosing onto each end of the diffuser, carefully but firmly and screw the sleeves over the connection points to ensure a snug fit and prevent leaking. 

Connect the CO2 
Locate the small connection point on the top of the diffuser. This is where the CO2 tubing will attach. Push the tubing firmly onto the connector to ensure a snug fit.

Common CO2 Problems for Aquarists and Fixes?

Low CO2 Readings

• Increase the bubble rate
• Check for any blockages or leaks in the diffuser and the setup that may be preventing proper CO2 flow
• Ensure that your diffuser is clean. If you notice any algae buildup, soak it in a bleach solution for half an hour, then rinse it in a dechlorinated water solution to remove any chlorine
• Check your CO2 bottle is not empty

High CO2 Readings  

• Decrease the bubble rate and monitor water parameters, perform a big water change if fish are gasping at the surface as they could be suffering CO2 poisoning which is fatal without quick action.
• Always start with low bubbles per second and build up to avoid issues

Algae Bloom / Plants not Responding

• The CO2 levels may not be set optimally.
• If you're consistently seeing a green drop checker, it might be due to other limiting factors, such as nutrient deficiencies. Plants grow faster with CO2 and will deplete nutrients more quickly.
• BBA can occur in areas of high flow and/or high CO2 misting, this is common on slow-growing plants, ensure the CO2 bubbles aren't being directed to one confined area of the tank and there is a steady even flow around the tank.

Top Tip

Set your CO2 to turn on two hours before the lights come on. This allows the CO2 to saturate the water, enabling the plants to perform photosynthesis with optimal levels of CO2 available when the lights are on. The drop checker should be green or close to green as the lights come on.

Turn the CO2 off one hour before the lights go off. 

Regular maintenance and monitoring will help prevent these issues and ensure a healthy aquatic environment.

Cost of CO2 on an Aquarium

Depending on the size bottle and the type of regulator you opt for, the cost can range between approximately £200-£250 for everything you need. However you will have ongoing CO2 refill costs, this cost will depend on how fast you go through CO2, however a 2L bottle of CO2 on a ADA 90p for example could last up to 3 or 4 months give or take a couple of weeks depending on how heavily planted it is and how much you inject.

We hope this helps you feel less worried and more knowledgeable when it comes to installing CO2 on your aquarium, if you have any issues please feel free to discuss it with us in store, we are here to help you get the best from your aquarium!