Mastering Aquarium CO₂: How to Measure, Maintain, and Maximise Plant Growth Safely

Adding CO₂ to your planted aquarium can dramatically improve plant health, colouration, and growth—but getting it right is key. In this guide, we’ll break down the most asked questions on aquarium CO₂, helping you optimise your tank safely and effectively.

Why Add CO₂ to a Planted Aquarium?

CO₂ (carbon dioxide) is essential for photosynthesis. In a planted tank, especially one with medium to high light, CO₂ injection is often necessary to avoid nutrient imbalances and algae outbreaks. With the right setup, your plants will thrive with vibrant colours and lush growth.

How Much CO₂ Is Safe in an Aquarium?

Most aquascapers aim for 20–30 ppm (parts per million) of CO₂ in a planted aquarium. This range promotes healthy plant growth without endangering fish or invertebrates. More than 35 ppm can cause fish stress or suffocation.

Signs of too much CO₂:

• Fish gasping at the surface

• Unusual lethargy

• Drop checker turns yellow (more on that below)

How much CO2 should I inject into my Aquarium / How many bubbles per second?

There’s no one-size-fits-all answer because CO₂ needs depend on:

• Tank size

• Plant density

• Water surface agitation

• Filtration flow

• Light intensity

However, here are general BPS starting points you can use:

Always start low and adjust gradually while monitoring your CO₂ level with a drop checker, its a good idea if you are setting up CO2 on a tank for the first time, to do it on a day where you're able to monitor it throughout the day, just to make sure its all safe and working correctly. 

How to Know If It’s Working

• Drop Checker: Should turn lime green, it may take an hour or so to change fully (indicates ~30 ppm CO₂)

• pH Drop: A drop of 1.0 from your tank’s degassed pH means you’ve likely reached around 30 ppm CO₂

• Observe Livestock: Gasping fish or stressed shrimp? Dial it back.

• Plant Growth: Look for pearling (tiny oxygen bubbles) and richer colouration.

Tips for CO₂ Injection:

• Inject CO₂ 1–2 hours before lights on, and turn off 1 hour before lights go out.

• Combine with good circulation so CO₂ reaches all areas of the tank.

• Use a high-quality diffuser for maximum micro-bubble diffusion and efficiency.

How to Measure CO₂ in Your Aquarium

a) Drop Checker Method
A drop checker is a simple and affordable tool that visually indicates CO₂ levels by changing colour based on concentration. It’s one of the most reliable and user-friendly methods for monitoring CO₂ in aquariums. While it typically takes around an hour to reflect changes, it provides a clear, ongoing visual reference. At Horizon Aquatics, we always recommend using a drop checker — it's the most effective way to ensure your CO₂ levels stay in the optimal range for healthy plant growth and happy livestock.

• Blue = low CO₂

• Green = ideal CO₂ (approx. 20–30 ppm)

• Yellow = too much CO₂

b) pH/KH Chart
You can estimate CO₂ using your water's pH and KH values. Use online calculators or charts that estimate CO₂ levels based on the carbonate hardness and pH reading. This method is okay as an estimate, however it assumes pH changes are caused only by CO₂, ignoring the effects of other acids like tannins or organic waste. It also doesn’t account for non-carbonate buffers that can skew KH readings, leading to inaccurate CO₂ estimates.

c) CO₂ Monitoring Systems
Advanced digital controllers can monitor and adjust CO₂ output based on real-time pH levels, this method is precise but can be expensive.

How to Choose the Right CO₂ System: DIY vs Pressurised

DIY CO₂ Systems
Typically uses a yeast/sugar mixture to produce CO₂.

• Pros: Low cost, can make at home

• Cons: While DIY CO₂ systems may seem like a budget-friendly option, they come with major drawbacks that make them unreliable for most aquariums. Output is inconsistent due to the fermentation process, making it difficult to maintain stable CO₂ levels — which can lead to plant stress and algae issues. There's little to no control over the rate of diffusion, no way to accurately count bubbles, and you can't attach a solenoid to safely switch the system off at night. DIY setups also require constant maintenance, with mixtures typically lasting just a week or two. And although some DIY kits offer more advanced features, they often approach the cost of a pressurised system without the precision, safety, or convenience. For any aquarist serious about plant health and long-term stability, a proper CO₂ system is well worth the investment.

Pressurised CO₂ Systems
The gold standard for stable, reliable CO₂ delivery. Ideal for most medium to large tanks.

• Pros: Precise control, consistent delivery, works with timers and solenoids

• Cons: Higher upfront cost, but long-term value

Key Components of a Pressurised System:

• CO₂ Cylinder

• Regulator (preferably dual-stage)

• Solenoid valve (optional but useful for timers)

• Bubble counter with check valve

• Diffuser (glass or inline for fine mist diffusion)

• Drop checker

What are the Different ways to inject CO2 into your Aquarium?

Choosing the right CO₂ injection method depends on your tank size, setup, and aesthetic preferences. Here are the most common options:

 In-Tank Diffuser (Glass or Acrylic)

Pros

• Ideal for smaller aquariums
• Produces fine micro-bubbles directly in the tank
• Easy to install and visually minimal

Cons

• Bubbles are visible, which may not suit every aquascaper

Inline Diffuser

Pros

• Installed on the outflow of an external filter
• Keeps the tank clean and clutter-free
• Excellent diffusion for larger tanks

Cons

• Requires external canister filter and careful installation

CO₂ Reactor

Pros

• Very efficient at dissolving CO₂
• No visible bubbles in the tank
• Great for large, high-demand planted tanks

Cons

• Bulkier equipment, typically hidden in the cabinet

What is Gas Exchange in Aquariums?

Understanding Gas Exchange in Aquariums (and in Nature)

Something that is often overlooked when installing CO2 on your aquarium, and something that is vital, is surface agitation.

Gas exchange is the process through which gases like oxygen (O₂) and carbon dioxide (CO₂) move between the water and the atmosphere. This exchange is critical for aquatic life and maintaining chemical stability in both natural bodies of water and aquariums.

How It Happens in Nature

In lakes, rivers, and ponds, gas exchange occurs primarily at the air-water interface — the surface of the water. Natural processes such as wind, current, and wave action constantly agitate the water surface, increasing the surface area and improving diffusion. This allows oxygen from the atmosphere to dissolve into the water and lets excess CO₂ escape.

Photosynthesis by aquatic plants and algae also plays a role, especially during daylight. Plants absorb CO₂ and release O₂ during photosynthesis, further enriching the water with dissolved oxygen. At night, however, plants switch to respiration, consuming oxygen and releasing CO₂ — just like animals.

Gas Exchange in Aquariums and how to Increase it

In a closed aquarium system, the natural mixing processes seen in rivers and lakes are absent or limited. This makes mechanical intervention essential to promote gas exchange. Key methods include:

• Surface agitation (via filter outflow, powerheads, or air stones)

• Surface skimmers (to prevent biofilm build-up that restricts gas exchange)

• Proper tank circulation (to move oxygen-rich water across all levels)

Without sufficient surface agitation or oxygenation, CO₂ can accumulate (especially in CO₂-injected planted tanks), leading to acidic pH drops and oxygen depletion, which can cause fish stress, laboured breathing, or death.

CO₂ & O₂ Balance

Maintaining the correct balance between CO₂ and O₂ is crucial:

• Too much CO₂ can lead to hypoxia (low oxygen), particularly at night when plants consume oxygen.

• Too little CO₂ in planted tanks can limit photosynthesis and plant growth.

A well-oxygenated tank ensures that fish and beneficial bacteria have the oxygen they need, while still maintaining appropriate CO₂ levels for plant health.

Key Takeaway

Gas exchange is the invisible engine that powers a healthy aquatic system. By mimicking natural water movement and ensuring consistent surface agitation, aquarists can maintain optimal oxygen levels, prevent CO₂ buildup, and create a stable, thriving ecosystem for both fish and plants.

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Final Tips for Success

• Always start low and increase slowly to avoid shocking livestock.

• Use a reliable diffuser that produces micro bubbles for better absorption.

• Combine CO₂ injection with good lighting and fertilisation for best results.

• Turn off CO₂ at night (using a solenoid on a timer).

Adding and managing CO₂ doesn’t have to be complicated. With the right tools and approach, you can transform your planted tank into a vibrant, healthy aquascape. Whether you’re just starting or fine-tuning your setup, understanding how to measure and maintain ideal CO₂ levels is the key to long-term success.

Watch our Complete Guide to Installing CO2