CO2 System

A pressurised CO2 system is the most transformative equipment upgrade for a planted aquarium. Carbon dioxide is a fundamental building block of photosynthesis, and in the closed environment of an aquarium, it is almost always the limiting factor for plant growth. Injecting CO2 unlocks explosive growth, vivid colouration, and the ability to grow demanding species that are impossible without supplementation.

System Components

A complete pressurised CO2 system consists of:

1. CO2 Cylinder

A refillable steel or aluminium tank, available in sizes from 0.5kg to 5kg. Larger cylinders last longer between refills.

• Tip: A 2kg cylinder typically lasts 3–6 months on a 60cm tank.

2. Regulator with Solenoid Valve

The regulator reduces the high cylinder pressure (~800 psi) to a safe, usable working pressure (~20–40 psi). The solenoid valve is an electromagnetic on/off switch controlled by a timer.

• Dual-Stage Regulator: Prevents "end-of-tank dump" (a dangerous pressure spike when the cylinder runs low). Highly recommended.

3. Needle Valve

A precision flow control valve built into or attached to the regulator. This allows you to fine-tune the exact bubble rate delivered to the tank.

4. Bubble Counter

A small inline chamber filled with water that lets you visually count the CO2 bubbles per second, providing a rough gauge of injection rate.

5. Tubing

CO2-resistant tubing (typically clear or black polyurethane) connects the regulator to the diffuser. Standard silicone tubing is CO2-permeable and should be avoided.

6. Check Valve

A one-way valve that prevents water from back-siphoning into the regulator — a critical safety component.

7. Diffuser

Dissolves the CO2 into the water. Covered in detail in our dedicated CO2 Diffuser article.

Setting Up & Tuning

1. Install the regulator onto the cylinder. Ensure all connections are tight.
2. Connect CO2-resistant tubing from the needle valve through the bubble counter, check valve, and finally to the diffuser.
3. Set Timer: Run CO2 1–2 hours before lights on and turn off 1 hour before lights off.
4. Adjust the needle valve to achieve 1–3 bubbles per second (varies by tank size).
5. Monitor with a drop checker — target a lime-green colour indicating ~30ppm CO2.

Safety Considerations

• Ventilation: Excessively high CO2 in poorly ventilated rooms can cause drowsiness. Always ensure good room airflow.
• Fish Safety: CO2 above 30ppm can stress or harm fish. Always use a drop checker and monitor livestock behaviour.
• End-of-Tank Dump: Use a dual-stage regulator to prevent uncontrolled gas release when the cylinder empties.

Trusted Brands

• CO2Art: Excellent regulators, complete kits, and accessories at fair prices.
• ADA: Premium Advanced CO2 System with impeccable build quality.
• UNS: Affordable and reliable regulators for beginners.
• Fzone: Budget-friendly but functional options.

Pros & Cons

Advantages

• Plant Growth: The single biggest boost to plant health, growth rate, and colour.
• Algae Prevention: Healthy, fast-growing plants outcompete algae.
• Species Range: Enables growth of demanding carpeting plants and vivid reds.
• Control: Precise regulation of injection rate and timing.

Considerations

• Cost: Initial setup (cylinder + regulator + diffuser) is a significant investment.
• Ongoing Cost: Cylinders need periodic refilling.
• Complexity: Requires tuning and monitoring, especially when starting out.
• Risk: Overdosing CO2 can harm livestock.