Aquarium CO2 Calculator
Calculate dissolved CO2 levels in your planted aquarium from pH and KH readings. Free online CO2 calculator with reference table and injection recommendations.
CO2 is below optimal for planted tanks. Consider CO2 injection for better plant growth.
CO2 Injection Recommended
Your CO2 level is below 15 ppm. For a planted tank, we recommend pressurized CO2 injection to reach the 20-30 ppm sweet spot. For a 20-gallon tank, target approximately 2.0 bubbles per second to achieve ~30 ppm.
pH / KH / CO2 Reference Table
| pH | 2 dKH | 4 dKH | 6 dKH | 8 dKH | 10 dKH |
|---|---|---|---|---|---|
| 6.0 | 60.0 | 120.0 | 180.0 | 240.0 | 300.0 |
| 6.4 | 23.9 | 47.8 | 71.7 | 95.5 | 119.4 |
| 6.6 | 15.1 | 30.1 | 45.2 | 60.3 | 75.4 |
| 6.8 | 9.5 | 19.0 | 28.5 | 38.0 | 47.5 |
| 7.0 | 6.0 | 12.0 | 18.0 | 24.0 | 30.0 |
| 7.2 | 3.8 | 7.6 | 11.4 | 15.1 | 18.9 |
| 7.4 | 2.4 | 4.8 | 7.2 | 9.6 | 11.9 |
| 7.6 | 1.5 | 3.0 | 4.5 | 6.0 | 7.5 |
Green values (20-30 ppm) are ideal for planted tanks. Yellow values indicate suboptimal levels. Red values (>40 ppm) are dangerous for fish.
Recommended CO2 & Lighting Equipment
Fluval Plant 3.0 LED
$$$Full spectrum, programmable, great for planted tanks
How It Works
This calculator uses the standard carbonate equilibrium formula: CO2 (ppm) = 3 × KH (dKH) × 10^(7 − pH). This relationship exists because dissolved CO2 forms carbonic acid in water, which interacts with the carbonate buffer system measured by KH.
Enter your tank volume, measured pH, and KH (carbonate hardness in dKH). The calculator determines your dissolved CO2 concentration and tells you whether it falls within the ideal 20-30 ppm range for planted tanks.
The reference table shows CO2 values across common pH and KH combinations, color-coded for quick identification. Green indicates ideal planted tank levels, yellow is suboptimal, and red indicates dangerous levels for fish.
Bubble rate estimates are based on approximately 1 bubble per second per 10 gallons to achieve ~30 ppm, though actual results vary with diffuser type and surface agitation.
Frequently Asked Questions
How is dissolved CO2 calculated from pH and KH?
Dissolved CO2 is calculated using the formula: CO2 (ppm) = 3 × KH (dKH) × 10^(7 − pH). This relationship comes from the carbonate buffering system in water. As CO2 dissolves, it forms carbonic acid which lowers pH, so measuring both pH and KH lets you determine the CO2 concentration.
What is the ideal CO2 level for a planted aquarium?
The ideal CO2 level for planted aquariums is 20-30 ppm. At this range, most aquarium plants photosynthesize efficiently without stressing fish. Below 15 ppm, plants grow slowly and algae tends to outcompete them. Above 40 ppm, fish may show signs of distress like gasping at the surface.
How many bubbles per second should I set my CO2 regulator to?
A general starting point is approximately 1 bubble per second per 10 gallons of aquarium water to reach around 30 ppm. However, this varies based on surface agitation, plant mass, and diffuser efficiency. Always use a drop checker or pH/KH test to verify actual CO2 levels rather than relying solely on bubble count.
Can I use liquid carbon instead of CO2 injection?
Liquid carbon products like Seachem Excel provide an alternative carbon source for plants but are less effective than pressurized CO2. They work well for low-tech planted tanks with undemanding plants. For high-light, high-growth setups with demanding plants like carpeting species, pressurized CO2 injection is strongly recommended.