How to Set Up an Aquarium CO2 System the Easy Way

When it comes to planted tanks, we always encourage beginners to start with easy, slow-growing plants that only need low lighting and an all-in-one fertilizer. However, certain plants are a little more difficult to grow underwater and may require high lighting and extra carbon dioxide (CO₂) beyond what is naturally provided in the air. When it comes to injecting CO₂ gas into water, aquarists have tried all kinds of techniques, types of equipment, schedules, and dosage amounts to put together their own custom DIY systems. At Aquarium Co-Op, we have experimented with many of them and put together this basic guide to explain our favorite method that is reliable and easy to use.

Does CO2 get rid of algae? It’s a common belief that CO₂ automatically fixes algae problems, but this is not true. A healthy planted tank must have three components in balance — lighting, fertilizer, and carbon dioxide. CO₂ is just one of the primary nutrients that plants need to grow. Many beginners use too much light and fertilizer, so adding CO₂ can help balance the aquarium. However, if a tank has high lighting and CO₂ injection but, for example, too little fertilizer, then algae will appear because of the imbalance.

Let’s use a cookie recipe as an analogy. If you add 5x the usual amount of flour (e.g., fertilizer) to your dough, then you must also multiply the rest of the ingredients (e.g., lighting and CO₂) by 5x, which will result in a bigger batch of cookies (e.g., greater plant growth). However, if you add 5x the amount of flour and then try to “balance” the recipe by only adding 5x the amount of chocolate chips (e.g., CO₂), the rest of the ingredients are not in the correct ratio and will result in a bad cookie (e.g., algae growth).

Do all aquarium plants need CO2 injection? As mentioned before, all aquatic plants use CO₂ as one of their basic building blocks. Some types like cryptocoryne plants do not need the extra CO₂, while other plants like scarlet temple could benefit from it but don’t require it. A third category of plants — which includes Blyxa japonica, dwarf hairgrass, and dwarf baby tears and other similar carpeting plants — has higher demands and necessitates the use of CO₂ for the best chances of success. 

Materials for a CO₂ System

In this guide, we are focusing on the installation of the CO₂ system and not the lighting and fertilization components. To get started, gather the necessary equipment and tools:

1. CO₂ regulator
   
   • What is a regulator? A regulator is a device that allows you to precisely control how much gas exits the CO₂ cylinder tank and enters the aquarium water.
   • What is the difference between a single-stage vs. two-stage regulator? A single-stage regulator reduces the cylinder’s gas pressure in one step, whereas a two-stage regulator reduces the pressure in two steps, resulting in a more stable and reliable flow of CO₂. A two-stage regulator also helps to prevent “end-of-tank dumps,” in which a nearly empty CO₂ cylinder may dump out the rest of its gas in one go.
   • Should I use a DIY CO₂ or pressurized CO₂ system? We have tested many types of DIY systems using yeast, citric acid, and other mixtures, and while they may be cheaper, they are not as stable as a pressurized CO₂ system using a regulator and cylinder. The DIY reactions often make lots of CO₂ at the beginning and then decline over time, and the inconsistent amounts of CO₂ can make it difficult to balance a planted tank. Furthermore, the pressure is not as high, temperature can affect the reaction, and the overall process is time-consuming to maintain. With a pressurized system, we just set it up once and let it run for one to three years before having to refill the cylinder.

2. Aquarium Co-Op manifold block add-ons (optional)
   
   • You can install up to five extra manifold blocks add-ons to expand the system and run CO₂ to multiple tanks.

3. CO₂ cylinder tank
   
   • Can I use a CO₂ paintball cylinder? No, the Aquarium Co-Op regulator is not intended for use with paintball tanks. They work with standard cylinder tanks that have the male thread size CGA320.
   • Where can I buy a CO₂ cylinder? We like to get ours from local home brewing supply stores and welding supply stores. Usually, they also offer CO₂ refill services if you bring back your cylinder when it’s empty.
   • What size CO₂ cylinder should I get? If you are running a high tech planted aquarium injected with high amounts of CO₂, people recommend getting the largest size possible so you will not have to refill the cylinder as frequently. However, for the average customer, we often suggest a 2.5–5 lb. cylinder for 20-gallon aquariums or smaller, a 5 lb. cylinder for 25- to 40-gallon aquariums, and a 10 lb. cylinder for 55-gallon aquariums or larger. If you plan on using one regulator with five or six aquariums, then scale the cylinder size accordingly.

4. Airline tubing or CO₂ tubing
   
   • Do I need to use special CO₂ proof tubing or CO₂ resistant tubing? We use the Aquarium Co-Op airline tubing (i.e., a flexible, black tubing made from food-grade PVC) on all of our aquariums and have not detected any perceptible loss of CO₂. In our experience, special CO₂ tubing is more expensive, harder to bend, and not as readily available.

5. Regular check valve or stainless steel check valve (optional)
   
   • Do I need a check valve for my CO₂ system? Check valves are used to prevent water from flowing out of the aquarium and pouring all over the regulator when it is turned off. The bubble counter in the Aquarium Co-Op regulator comes with a built-in check valve, but you can install a second one as backup if desired. We have personally used the regular plastic check valves with CO₂ systems at our fish store, warehouse, and homes, and they have not broken down. That being said, CO₂ does degrade plastic after a very long time, so we also offer a stainless steel version for greater durability.

6. CO₂ diffuser
   
   • Which type of diffuser should I get? Any CO₂ diffuser intended for aquariums that can operate at approximately 40–50 psi should be fine.
   • How do I clean a CO2 diffuser if it becomes clogged? Diffusers must be cleaned or replaced at some point because of algae buildup. Because diffusers can be made of different materials, follow the manufacturer’s cleaning instructions to use diluted bleach, vinegar, or other methods.

7. Water or mineral oil
   
   • Regular tap water can be used to fill the bubble counter so that you can see the approximate rate at which CO₂ is entering the aquarium. However, the water will evaporate over time, so mineral oil can be used instead so you never have to refill the bubble counter.

8. Electrical outlet timer
9. Adjustable wrench with at least 1.25-inch width
10. Scissors
11. Spray bottle with water and a few drops of Dawn dish soap

How to Install a CO₂ System

To help you visualize the entire CO₂ system, this high-level diagram shows how all the parts are connected:

• The regulator (B) screws onto the CO₂ cylinder (A).
• Optional manifold block add-ons can be added to the regulator (B).
• The bubble counter (C) on the regulator is filled with liquid, and airline tubing is attached to the lid of the bubble counter.
• The airline tubing connects to the diffuser (D), which is placed at the bottom of the aquarium.
• The optional check valve (E) is installed in line with the airline tubing near the aquarium rim.
• The regulator’s solenoid valve cable (F) is connected to the power adapter (G).
• The power adapter (G) plugs into the electrical outlet timer (H), which plugs into a wall outlet or power strip.

Is it bad if the CO₂ bubbles from the diffuser are reaching the water surface? No, this is normal. The key is to place your diffuser as low as possible in the aquarium. When the bubbles are released from the diffuser, they imperceptibly get smaller and smaller as they rise and the CO₂ gas is being absorbed into the water.

Place the diffuser at the base of the aquarium to give the CO₂ bubbles a longer time to dissolve into the water.

How Much CO₂ to Dose

In the manual, we recommend tuning the regulator to approximately 1 bubble per second (i.e., the rate of CO₂ bubbles flowing through the bubble counter) because we would rather start with a lighter amount of CO₂ to keep the fish safe. That being said, CO₂ dosing amounts are different for every tank, and the bubble rate is not a perfect form of measurement since each aquarium has different plant and fish stocking levels. Also, we personally do not use drop checkers to chase the “perfect” amount of 30 ppm of CO₂, but instead we let nature and the plants tell us when they are happy.

When the plants photosynthesizing during the daytime, they consume carbon dioxide and produce oxygen (O₂) and sugars as a byproduct. If the plants have enough light and carbon dioxide, they can produce so much oxygen that it saturates the water and you can visibly see small bubbles released from their leaves. In our warehouse, we dial the CO₂ level on our plant-holding aquariums until we consistently see this “pearling” effect. Because plants are living things, it usually takes at least 24 hours after we adjust the CO₂ to see any effect, so we like to wait three days before making the next change to the system.

Aquatic plants “pearl” or visibly produce bubbles when the water is saturated with oxygen.

When should I turn on and off the CO₂ in my aquarium? As mentioned before, plants use CO₂ when there is light to photosynthesize. However, the process reverses at night and becomes the respiration cycle, in which plants consume oxygen and sugars and release CO₂. Therefore, we want to shut off the CO₂ regulator when the aquarium light is off. For more optimized CO₂ usage, program the regulator’s timer to turn on 1–2 hours before the light comes on and turn off 1 hour before the light shuts off. (If you only have one timer, you can use the same timer with a power strip so that the light and regulator turn on and off at the same time.)

Is CO₂ dangerous for aquarium fish? It can be harmful for animals in large enough quantities if (1) CO₂ causes the water pH to drop too quickly or (2) people try to be so efficient with the CO₂ that they end up cutting off the oxygen that fish need to breathe. In the latter case, some hobbyists try to minimize surface agitation so that less gas exchange occurs and less CO₂ escapes the water. However, less gas exchange also means less oxygen will enter the water, which can cause your fish to struggle and gasp for air. Our recommendation is to increase both CO₂ and O₂ in the water by using an air stone (or other device that agitates the water surface) in conjunction with your pressurized CO₂ system. Yes, you may have to increase your bubble rate a little to compensate for the slight loss of CO₂, but having enough oxygen for your fish (and plants at night) is more important and can help lead to the pearling effect that is so desired by planted tank enthusiasts.

Best of luck with your new pressurized CO₂ system and we hope you have fun exploring the world of high tech plants!