CO2 and Planted Aquariums: Should You Add CO2 to Your Tank?

The single most important nutrient for plants is carbon dioxide (CO2). Half of a plant’s mass is carbon, and so without sufficient CO2 in your aquarium water, plant growth will inevitably be stunted.

CO2 in planted aquariums comes from three sources:

  • The air, via gas exchange at the water surface, just as with oxygen
  • Respiration by fish and other life, including plants when not lit
  • Deliberate CO2 fertilization to encourage plant growth

What is the optimal level of CO2 in planted aquariums?

Fish don’t require high levels of CO2, and in fact, carbon dioxide is toxic to fish above levels of 30-35mg/l.

In well set-up aquariums, excessive CO2 is only usually a problem if there is an overdose from CO2 fertilization. An over-stocked aquarium with poor water circulation can also suffer from CO2 build up to dangerous levels, however, especially at night. Oxygen can also become depleted; watch out for fish gasping at the surface.

High levels of CO2 are no problem for the vast majority of plants. Rather, the problem aquarium plants typically face is a lack of carbon dioxide. Too little CO2 is usually a recipe for poor plant growth, however great the tank’s substrate or lighting. In such circumstances, algae will often proliferate, as it grabs the nutrients and exploits the lighting ahead of the stunted plants.

Keeping both plants and fish happy in the same aquarium means balancing the CO2 requirements of the plants with the limits of the fish to create a healthy ecosystem.

Testing the CO2 level

Aquarists don’t usually measure the CO2 level directly. Instead, we work out the pH and KH levels of the aquarium water and cross-reference with a chart to determine the CO2 concentration.

Another popular method of monitoring CO2 levels is to use a CO2 constancy device or drop checker. These employ a calibrated tracing fluid that changes color as the pH fluctuates with changing CO2 levels. They sit permanently inside the aquarium, and so give an at-a-glance CO2 reading:

  • blue = too little CO2
  • green = optimal CO2
  • yellow = excess CO2

Insufficient CO2

As a rule of thumb, a typically planted aquarium requires about 1 gram of CO2 per 25 gallons (100 liters) per day. Without sufficient CO2, plants will grow slowly, even with good lighting, a rich substrate, and the addition of other nutrients. Algae will often become a problem.

To remedy this problem, plant enthusiasts usually fertilize their aquarium with extra CO2. The aim is to ensure a CO2 level of between 15-35mg/l.

Methods of increasing the CO2 level, in order of preference:

  • Aerosols, liquids or tablets that release CO2 into the water (I like this one)
  • A DIY or shop-bought yeast-based CO2 reactor and diffuser
  • A pressurized CO2 system, with solenoid, bubble counter and tubes

I don’t recommend the first option for anything but a short-term solution.

Yeast-based CO2 systems work well in smaller tanks, though they do require some maintenance. You can buy them for £20 ($30), but they’re easy to make, especially if you don’t care too much what they look like.

Pressurized CO2 systems are the best choice for larger planted aquariums. They enable you to control very precisely how much CO2 enters the tank, and unlike with a yeast-based system, the supply is completely consistent. The downside is mainly cost, with prices starting at around £100 ($150). The other problem is the general hassle – more technology always means more things going wrong.

Yeast-based CO2 systems are very unlikely to overdose the tank, but you should test the CO2 level regularly if you want to understand the tank conditions as much as possible. Pressurized systems can easily overdose a tank and kill fish, so install a drop checker and keep an eye on it.

Note you only need to supply CO2 at night. Pressurized systems usually include a solenoid, so the supply is automatically cut off when the lights go out. There’s no easy way to stop yeast-based systems producing CO2, but they are much less potent anyway.

Excessively vigorous tank filtration will drive the CO2 you’re going to so much effort to inject straight out of the water. Air stones in particular have no place in planted aquaria.

Don’t add extra fish to try to increase CO2 levels! In the night when the plants are also respiring, the CO2 can build up to high levels, not to mention the risk of oxygen depletion and general pollution problems.

If there’s too much CO2 in the water, reduce the fertilization rate according to the manufacturer’s instructions.

Is adding CO2 essential for planted aquariums?

In a word, no. If you have have softish water and green fingers, you can grow plants in pretty much any aquarium provided there’s sufficient light and other nutrients, and you pick the correct species.

Some aquarists even make a virtue out of so-called Natural tanks, where they concentrate on trying to create balanced ecosystems with the minimum of technology and interference. One system, sometimes called ‘El Natural’, even discourages water changes in an effort to try to establish a self-regulating organic system.

But for the typical freshwater aquarist who wants a pretty tank with growing plants, fish and no plague algae, adding CO2 via a yeast-based system offers a good balance of effort to reward.

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