Carbon Dioxide is fundamental to all plant growth, as carbon is a building block of life. Use it in your aquarium and your plants will thrive. As far as plants are concerned, most aquariums are deficient in CO2. Give them extra CO2, and as long as they also have good light, and other nutrients, they will grow big and strong, and stay healthy. 

But Carbon Dioxide products vary in price, complexity and effectiveness, and it’s important to choose the right one for you and your plants. We explain all the different options below:

Aerosols

Aerosol CO2 sets are perhaps the cheapest and quickest way to add CO2 to an aquarium. They consist of a pressurised CO2 can, a hose, and a diffuser. Press the button on the can and the diffuser fills with CO2 gas. When in contact with water, the gas is soaked up from inside the diffuser chamber until the gas is gone, and the diffuser is full of water. Press the can again, fill the diffuser again, and the whole process is repeated. 

Although cost-effective at point of purchase, aerosol sets have a few drawbacks. The diffuser doesn’t diffuse the gas into the water as well as a ceramic diffuser or ladder diffuser does, and if you don’t manually refill the diffuser, the aquarium will go without CO2 until you do. Fluctuating CO2 levels and poorly circulated CO2 are often linked to algae problems in planted aquariums. 

Aerosols will only increase CO2 significantly in small to medium-sized aquariums, and the can will run out quite quickly. So they are better than no CO2, but as not as good as other methods of CO2 injection.

Yeast based systems

Yeast based CO2 systems are cost-effective to buy and run at fairly low pressure. Yeast plus sugar and water equal alcohol and CO2, so these systems come in kit form with sachets and a screw-top canister, and you actually make up a fluid that then ferments and produces carbon dioxide gas. CO2 is then introduced to the aquarium via a ladder style diffuser. 

For best results yeast-based systems need to be in a warm room, and the cycle of CO2 production ranges from none at all when first set up, to a few bubbles, then quite a few bubbles, before the bubble production ceases altogether. At this point, the user should rinse the canister and start again, with replacement sachets being available separately. 

Yeast based systems are a good introduction to CO2 and suitable for beginners, but like with aerosol systems, they are best for small to medium-sized aquariums and because CO2 production can change over time, the fluctuation in CO2 levels in the aquarium is not ideal. 

Pressurised CO2 systems

Pressurised CO2 is the system of choice for experienced aquatic plant growers the world over. Available with either disposable or refillable CO2 canisters, a set will consist of a pressurised gas bottle, a regulator valve, some hose, and a diffuser. 

Turn the knob on the gas bottle and the regulator will control how much CO2 is released via a precision needle valve. A basic regulator will have a pressure gauge displaying operating pressure, but a better reg will have two gauges, one displaying operating pressure and one showing the pressure in the gas bottle, which can also be used as an indicator of how much gas is left. 

Other equipment

Pressurised CO2 systems also come with optional extras. A solenoid valve can be placed inline to shut the gas off. Plug the solenoid into a plug-in timer and it can be set to turn CO2 on and off automatically, as plants actually produce CO2 at night, so don’t need any extra. A non-return valve will stop any back siphoning of aquarium water when gas is off, and adaptors are available to enable different thread size fitting if required. 

Different sizes of gas bottles are available, although a 500g bottle or larger is best, even for small to medium-sized aquariums, and tiny bottles can last just a few days. You’ll need a long term CO2 test kit, also known as a drop checker, to provide a visual indication of how much CO2 is dissolved in the water.

Diffusers can vary from a ladder or spiral type, to inline to ceramic. A high quality ceramic diffuser is generally regarded as the best, as it provides a fine mist, and the smaller the bubble, the easier it diffuses and is circulated by the water.

The dos and don'ts of CO2

Although carbon dioxide can be overdosed with plants, with no side effects, the same cannot be said about fish. Add too much CO2 to your aquarium and your fish may die, so it's another good reason to invest in a kit with a good, precision regulator, and a CO2 test kit. 

Airstones and venturis will drive off CO2, so don’t combine the two. And pressurised CO2 bottles are potentially dangerous in the wrong hands, so keep out of the reach of children. 

A consistent CO2 level of between 15-30mg/l is recommended for planted aquariums. Combine with plant growth-enhancing lighting, a nutritious planting substrate, and complete liquid fertiliser for best results.