CO2 for Planted Aquariums: Dosing Methods and Setups

Planted aquarium which requires co2 and ample lighting
Dennerle Scapers Tank 10 Gallon Aquarium Kit with LED Light Fixture

Everyone knows aquatic plants need proper nutrition to grow and develop colorful leaves. You’re probably familiar with essential plant nutrients like nitrogen, phosphorous, iron and copper which plants take in through their roots and leaves. These nutrients are converted into energy-rich protein, sugars, fats and starches. But what is the relationship with CO2 and planted aquariums? Carbon is one of at least 17 essential nutrients required by plants. Aquatic plants can use bicarbonate but the preferred carbon source is carbon dioxide (CO2).

Diagram of photosynthesis with CO2 in a planted aquarium
Photosynthesis: Light + CO2 + Plant = Oxygen

Some CO2 gas dissolves into the aquarium water from the atmosphere. Fish respiration and the breakdown of organic matter also add CO2 to the water.
The trouble is, the plants use CO2 faster than it can be replaced through natural processes. This is especially true when plants are kept under bright lighting. When plants are starved for CO2 they stop growing and decline in health. Even if your tank has great lighting, perfect water chemistry and a rich substrate, the plants won’t thrive without adequate carbon dioxide.
The good news is there’s a way to counteract CO2 deficiency in planted aquariums. Decades of experience has shown that bubbling carbon dioxide gas into the aquarium water raises the CO2 level and dramatically improves plant growth.

Ways to Dose

CO2 isn’t something you can dose once a week. It has to be continuously added to the water to provide a constant supply of carbon to the plants.
There are two ways to fertilize your plants with CO2:

One way is the biological fermentation method, using yeast and sugar. The yeast ferments consuming sugar, creating CO2 gas as a byproduct.

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Diagram of yeast consuming sugar to produce CO2 in the biological fermentation process

The Dennerle Bio CO2 set for planted aquariums uses fermentation inside a bottle utilizing a special slow-release sugar gel, which regulates the fermentation process and continuously produces CO2 for about 30 days. The gas is sent into the aquarium through a CO2 proof airline tube, bubble counter and diffuser. When the bottle starts to decline in production (seen through the bubble counter), you install a new fermentation bottle.

Dennerle biological fermentation set up for planted aquariums: bio gel, bubble counter and bubble diffuser
Dennerle biological fermentation set up: bio gel, bubble counter and bubble diffuser

Another way is to use CO2 kits with pressurized cartridges of CO2. The cartridges are pre-charged. A regulator valve controls the gas flow gas flow to the aquarium. A diffuser can be used with a pressurized system to create a fine stream of bubbles. The tiny rising micro-bubbles increase the transfer of CO2 into the water. When the cylinder is depleted, toss it in the recycle bin and hook up a fresh one.

Sample setup: CO2 Cylinder with Milwaukee CO2 Regulator, leading to Check Valve, and CO2 being dispersed into the tank by a Diffuser

If you’ve got a large planted aquarium, use a bright lighting system, or have plants that consume a lot of CO2, a traditional gas cylinder is the way to go. These are the same gas cylinders used for beverage dispensers. They’re available in 5 and 10-pound sizes, based on the weight of the filled cylinder. You’ll also need a CO2 regulator with a fine needle adjustment valve to set the gas flow rate, and a gas diffuser to make the CO2 to dissolve into the water. The great thing is that CO2 cylinders are easy to refill by just bringing it to your local beverage distributor – a 5-pound cylinder can last several months depending on your gas usage.

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Tip: If you’re building your own CO2 system, be sure to use CO2-compatible tubing. Regular airline will become brittle and leak gas. CO2-safe tubing will remain flexible and won’t leak gas through the wall of the tubing.

CO2 Levels in the Aquarium

It’s important to regulate the level of CO2 in the tank. Under bright lighting a level of 30-ppm is recommended. If your tank is dimly-lit go with about 15-ppm. Keep in mind that too much CO2 can stress your fish and invertebrates.

The biological yeast systems produce a relatively small amount of CO2, just enough for a gentle boost, so regulation and adjustment aren’t really needed. However, if you’re using pressurized gas, you’ll want to monitor the CO2. You can use a CO2 chart to determine an approximate level. Test the tank for pH and KH, then find the CO2 level on the chart.

Chart of the relationship between CO2, KH, and pH

A pH checker is another way to continuously monitor CO2. The pH checker sits in the tank. A pH-indicating solution senses the pH level in the tank based on gas diffusing into the checker. A green color indicates a safe level of CO2. Yellow means the CO2 is high. Blue means low.

Tip: No matter what method you use, make changes slowly. It takes several hours for the pH and CO2 levels to stabilize after each adjustment. Take your time when dialing in your system.

Final Thoughts

It’s never been easier to experience the benefits from CO2 fertilization. Once your CO2 system is running, you’ll be amazed how fast your tank responds, the way plants will color up and produce lush healthy growth – enjoy!