Getting your CO₂ levels just right is one of the most important—and delicate—steps in running a successful planted aquarium. Too little CO₂ and your plants won't thrive. Too much, and your fish could be at risk. This guide will teach you how to balance CO₂ injection to fuel healthy aquatic plant growth while keeping your fish safe and stress-free.
What’s the Ideal CO₂ Level in a Planted Aquarium?
The recommended CO₂ concentration in most freshwater planted tanks is around 30 parts per million (ppm). At this level, plants have access to the carbon they need for rapid growth without endangering fish or shrimp. Maintaining this balance requires accurate tools and consistent monitoring.
CO₂ levels above 40 ppm can lead to oxygen displacement and fish distress, while levels below 15 ppm may not support demanding plant species.
How to Measure CO₂ in Your Tank
1. Drop Checker
A drop checker is a visual CO₂ monitor that sits inside your tank. It contains an indicator solution that changes colour based on the CO₂ concentration. A green colour indicates a safe and optimal range (~30 ppm), while blue means too little and yellow indicates too much.
2. pH-KH Chart Method
If you know your tank's carbonate hardness (KH) and pH, you can calculate CO₂ levels using a CO₂ chart. However, this method can be skewed by other acids present in the water, making it less reliable than a drop checker.
3. Observation of Livestock
Fish gasping at the surface, lethargy, or erratic swimming may be signs of CO₂ overdose. Always observe your livestock after adjusting CO₂ levels to ensure their well-being.
Dialling In Your CO₂ System
Start your CO₂ at a low bubble rate—typically 1 bubble per second for a 100L tank. Increase the rate gradually over the course of several days while watching your drop checker and livestock.
Make small adjustments, wait 24 hours, and monitor colour changes and fish behaviour. Consistency is more important than speed when dialing in CO₂. Aim for stable green in the drop checker before proceeding to further adjustments.
Use a high-quality needle valve and regulator to ensure precise control, and position your diffuser near a filter outflow or circulation pump for even CO₂ distribution across the tank.
Timing: When to Run CO₂ Injection
CO₂ should only be injected during the lighting period, as plants only photosynthesise in the presence of light. Most aquascapers set their CO₂ system to start 1 hour before the lights turn on and stop it 1 hour before the lights go off. This ensures that plants have access to CO₂ when they need it most and prevents CO₂ from building up overnight.
What About Oxygen Levels?
While CO₂ is essential for plants, oxygen is crucial for fish. To maintain oxygen levels, ensure good surface agitation during the night when CO₂ is off. Using a gentle air stone or increasing flow at night helps oxygenate the tank and avoid fish stress.
During CO₂ injection, reduce surface agitation slightly to avoid degassing CO₂ too quickly, but never eliminate surface movement entirely.
Signs You’ve Dialled in CO₂ Correctly
Look for steady plant growth, vibrant colours, and pearling (oxygen bubbles forming on plant leaves). Fish should behave normally—active, alert, and not gasping at the surface. Algae should be under control, and your drop checker should stay lime green during light hours.
Final Thoughts
Properly dialling in CO₂ is both an art and a science. With patience, monitoring, and high-quality equipment, you can achieve the perfect balance for plant growth and fish safety. Always make changes gradually and trust your tools—especially your drop checker and your fish.
Explore our full range of CO₂ systems and accessories at Charterhouse Aquatics and start mastering your tank’s CO₂ balance today.
