Current CO₂
4.8 ppm
Target pH
6.60
Bubble Rate
10.0 bps
Common range: 20–35 ppm. Above 40 ppm can stress fish.
| Color | CO₂ Range | Status |
|---|---|---|
| Blue | <15 ppm | Too low for most plants |
| Green | 25–35 ppm | Optimal range |
| Yellow | >40 ppm | Dangerous for fish |
| pH | CO₂ (ppm) | Status |
|---|---|---|
| 7.6 | 3.0 | Low |
| 7.4 | 4.8 | Low |
| 7.2 | 7.6 | Low |
| 7.0 | 12.0 | Low |
| 6.8 | 19.0 | Low |
| 6.6 | 30.1 | Optimal |
| 6.4 | 47.8 | Danger |
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Inputs
Result
At KH 4 and pH 7.4, the tank currently has only 4.8 ppm CO₂. To reach 30 ppm, pH must drop to 6.60. Start at about 10 bubbles per second and adjust based on the drop checker.
Inputs
Result
A 55-gallon tank at KH 3 and pH 7.2 has about 5.7 ppm CO₂. Target pH of 6.48 delivers 30 ppm. The larger volume requires roughly 27.5 bps to maintain adequate levels.
Inputs
Result
This small low-light tank has 3.8 ppm CO₂ at pH 7.6 and KH 5. A modest target of 20 ppm requires only 3.3 bps and an 8-hour photoperiod.
CO₂ in ppm equals 3.0 times your KH (in dKH) times 10 raised to the power of (7.0 minus pH). At KH 4 and pH 7.4, that gives 3.0 × 4 × 10^(−0.4) = 4.8 ppm. Lowering pH to 6.6 at the same KH yields about 30 ppm.
| pH | CO₂ at KH 2 | CO₂ at KH 4 | CO₂ at KH 6 |
|---|---|---|---|
| 7.4 | 2.4 ppm | 4.8 ppm | 7.2 ppm |
| 7.0 | 6.0 ppm | 12.0 ppm | 18.0 ppm |
| 6.6 | 15.1 ppm | 30.2 ppm | 45.2 ppm |
| 6.4 | 23.9 ppm | 47.8 ppm | 71.7 ppm |
Most planted tanks thrive at 25–35 ppm of dissolved CO₂. Below 15 ppm, growth slows and algae can outcompete plants. Above 40 ppm, fish may show distress. A drop checker reading green confirms you are in the 25–35 ppm sweet spot.
| CO₂ Range | Drop Checker | Plant Growth | Fish Safety |
|---|---|---|---|
| <15 ppm | Blue | Slow | Safe |
| 25–35 ppm | Green | Optimal | Safe |
| 35–40 ppm | Yellow-Green | Fast | Watch closely |
| >40 ppm | Yellow | N/A | Danger |
A rough guideline is tank gallons times target CO₂ ppm divided by 60. A 20-gallon tank targeting 30 ppm needs about 10 bubbles per second. Start lower and increase by 1 bps per day while monitoring the drop checker.
Turn CO₂ on 1 hour before lights come on so levels peak when photosynthesis starts. Turn it off 1 hour before lights go off since plants stop using CO₂ in darkness. Running CO₂ at night wastes gas and can harm fish.
Yes. Surface agitation drives CO₂ out of the water (off-gassing). Keep surface movement minimal during CO₂ injection. A gentle ripple is fine for gas exchange, but a strong current from a HOB filter or powerhead can waste 30–50% of your injected CO₂.
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Last Updated: Jul 24, 2026
This calculator is provided for informational and educational purposes only. Results are estimates and should not be considered professional financial, medical, legal, or other advice. Always consult a qualified professional before making important decisions. UseCalcPro is not responsible for any actions taken based on calculator results.