Aquarium CO2 Calculator
Calculate CO2 levels in your planted aquarium using pH and KH measurements. Get recommendations for optimal plant growth.
Water Parameters
pH/KH/CO2 Relationship
CO2 dissolves in water to form carbonic acid, lowering pH. With your KH of 4 dKH, target pH of 6.6 will give you approximately 30 ppm CO2.
CO2 Level
Drop Checker Color
Blue-Green (low-medium)
Suggested Bubble Rate
2 BPS
Recommendation
CO2 levels are optimal. Maintain current injection rate.
CO2 Reference
Important Notes
- • Always measure pH and KH from the same water sample
- • This formula assumes no other acids are present in the water
- • Use a drop checker as a secondary CO2 indicator
- • Never inject CO2 at night when plants don't photosynthesize
Understanding CO2 in Planted Tanks
Carbon dioxide is essential for plant photosynthesis. In nature, CO2 comes from fish respiration, decomposition, and atmospheric exchange. In planted tanks, supplemental CO2 injection can dramatically improve plant growth and health.
CO2 Level Guide
- 0-10 ppm: Low-tech tank levels (no injection)
- 15-20 ppm: Light injection, undemanding plants
- 25-30 ppm: Optimal for most planted tanks
- 30-35 ppm: High-tech tanks, demanding plants
- 40+ ppm: Dangerous to fish - reduce immediately
CO2 Injection Methods
Pressurized CO2
- Most consistent method
- Adjustable bubble rate
- Higher upfront cost
- Best for serious planted tanks
DIY Yeast/Citric
- Low cost to start
- Inconsistent output
- Requires maintenance
- Good for small tanks
Liquid Carbon
- Easy to dose
- No equipment needed
- Less effective than gas
- Can harm some plants
Frequently Asked Questions
How does the pH/KH/CO2 relationship work?
CO2 dissolves in water to form carbonic acid, which lowers pH. KH (carbonate hardness) buffers against pH changes. The relationship is: CO2 (ppm) = 3 × KH × 10^(7-pH). Lower pH with the same KH means more CO2.
What CO2 level should I target?
For planted tanks, 25-35 ppm is optimal for most plants without stressing fish. Start at 20 ppm and increase gradually. High-tech planted tanks with demanding plants may target 30-35 ppm. Never exceed 40 ppm.
Why is my drop checker always blue/green/yellow?
Blue = not enough CO2 (under 20 ppm). Green = optimal (20-30 ppm). Yellow = too much CO2 (over 30 ppm). Drop checkers lag about 1-2 hours behind actual CO2 levels, so make adjustments gradually.
How accurate is the pH/KH calculation?
The formula assumes only CO2 affects your pH, which isn't always true. Other acids (tannins, organics) can lower pH without adding CO2. For most tanks the formula is accurate within 10-15%. Use a drop checker as verification.
Should I run CO2 24/7 or only during lights-on?
Only inject CO2 when lights are on. Plants only use CO2 during photosynthesis. At night, plants respire and produce CO2, so adding more can drop oxygen dangerously low. Use a timer or solenoid to automate this.
My fish are gasping at the surface - is it CO2?
Possibly! Fish gasping indicates low oxygen or high CO2. Turn off CO2 immediately, increase surface agitation, and do a water change if needed. CO2 above 40 ppm can stress fish; above 50 ppm is dangerous.
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