How to Adjust pH and KH in a Planted Aquarium with CO2

pH and KH are two essential parameters to ensure water stability in a planted aquarium. CO2 injection can affect these values, which is why it is crucial to understand their interaction. This guide explains how to test, interpret, and correctly adjust these parameters.

Why Are pH and KH Linked to CO2?

• pH (hydrogen potential) measures water acidity.
• KH (carbonate hardness) stabilizes pH by acting as a buffer.
• CO2 injected into the water forms carbonic acid, which lowers the pH.
• Low KH causes rapid pH fluctuations → dangerous instability.

Steps to Precisely Adjust pH and KH

1. Test KH with a drop test (target: 3 to 6 °dKH).
2. Measure pH before and after CO2 injection.
3. Use a KH/pH correspondence chart to estimate dissolved CO2 concentration (target: 20-30 mg/L).
4. Adjust CO2 flow so the pH gently decreases to a target value without exceeding a 1-point difference (e.g. 7.4 ➝ 6.4).

Practical Tips for Stable Adjustment

• Use a solenoid valve to synchronize CO2 injection with lighting.
• Measure pH in the morning and 1 hour after lights on to monitor CO2 effect.
• Stabilize KH before any CO2 injection.
• Add mineral salts if your water is too soft (KH < 3).
• Prefer drop tests over strips for better accuracy.

Common Mistakes to Avoid

• Injecting CO2 in water with too low KH (< 2 °dKH).
• Not measuring pH daily during the first days.
• Injecting too much CO2 without gradual flow adjustment.
• Using a solenoid valve without a suitable timer.

Conclusion

Mastering pH and KH is a key step to maintain a healthy and balanced planted aquarium. By understanding their interaction with CO2, you can create an optimal environment for your aquatic plants and avoid acidic imbalances. Test, adjust, and most importantly: be consistent.

Additional Articles

• Comparison of CO2 Systems for Aquariums
• Install a CO2 Kit with Fire Extinguisher
• DIY CO2: Citric Acid + Baking Soda Method
• Diffuse CO2 in Multiple Aquariums with a Splitter