Why CO₂ matters in a planted tank
Aquatic plants need carbon dioxide to grow, just like land plants. In a tank without CO₂ injection, water holds only 2 to 5 ppm, enough for slow-growing plants such as Java fern and Anubias. Fast-growing stem plants and carpeting plants under strong light need more, typically 20 to 30 ppm. But too much CO₂ suffocates fish, so knowing your level matters.
The KH and pH method
In water, CO₂ forms carbonic acid, which lowers the pH. The carbonate hardness (KH) buffers against that change. If carbonate is the main buffer, the relationship gives:
CO₂ (ppm) ≈ 3 × KH (dKH) × 10^(7 − pH)
The calculator also inverts it to show the pH to aim for to reach your target CO₂ at your KH:
Target pH = 7 − log₁₀(target CO₂ ÷ (3 × KH))
Quick reference
| KH | pH for about 30 ppm CO₂ |
|---|---|
| 2 dKH | 6.3 |
| 4 dKH | 6.6 |
| 6 dKH | 6.8 |
| 8 dKH | 6.9 |
Example
With a KH of 4 dKH and a pH of 6.8, the estimated CO₂ is about 19 ppm, within the range for most planted tanks. To reach 25 ppm at the same KH, aim for a pH of about 6.7.
Units of hardness
- 1 dKH (German degree) = 17.9 ppm (mg/L) of CaCO₃
- 1 French degree (°f) = 10 ppm of CaCO₃ = 0.56 dKH
- 1 meq/L = 2.8 dKH
When the formula misleads
The method assumes carbonate is the only buffer. Tannins from driftwood and leaves, peat, phosphate buffers and some substrates lower pH without CO₂, so the formula overestimates CO₂. A drop checker filled with a 4 dKH reference solution is a good cross-check: blue means low, green about 30 ppm, yellow too high.
Safe practice
- Turn CO₂ on about an hour before the lights and off about an hour before they go out.
- Increase CO₂ gradually, watching the fish.
- Keep some surface agitation for oxygen, especially at night.
- If fish gasp at the surface, switch off CO₂ and aerate immediately.