The pH and KH table
Each cell is the CO₂ in ppm for that pair of readings. The usual target for a tank with injected CO₂ is shaded.
| KH | pH 6.4 | pH 6.6 | pH 6.8 | pH 7.0 | pH 7.2 | pH 7.4 |
|---|---|---|---|---|---|---|
| 1 dKH | 12 | 7.5 | 4.8 | 3.0 | 1.9 | 1.2 |
| 2 dKH | 24 | 15 | 9.5 | 6.0 | 3.8 | 2.4 |
| 3 dKH | 36 | 23 | 14 | 9.0 | 5.7 | 3.6 |
| 4 dKH | 48 | 30 | 19 | 12 | 7.6 | 4.8 |
| 6 dKH | 72 | 45 | 29 | 18 | 11 | 7.2 |
| 8 dKH | 96 | 60 | 38 | 24 | 15 | 9.6 |
How the number is worked out
CO₂ dissolved in water turns part of itself into carbonic acid and lowers the pH. How far the pH falls depends on the KH, the carbonate that buffers it. The standard aquarium formula reverses that: CO₂ in ppm = 3 × KH in dKH × 10(7 − pH). Each step of 0.2 down in pH means about 60 percent more CO₂, so a small error in the pH reading moves the answer a lot. Use a pH meter or a fresh test, and test pH and KH from the same sample.
When the answer is wrong
The formula assumes carbonate is the only thing holding the pH. Driftwood and leaves release tannins, active planted-tank soils pull the KH down and some fertilizers and pH buffers add phosphate: each makes the calculated CO₂ read higher than it is. If your tank has any of these, check the result against a drop checker, a small glass bulb of 4 dKH reference water that turns from blue to green at about 30 ppm, or against the fish: gasping at the surface means too much, whatever the number says. Fish gasping
Turning it up safely
Raise the bubble rate a little at a time, over several days, and watch the fish for an hour after the CO₂ comes on each morning. Keep the surface rippling so oxygen gets in, run the CO₂ on a timer that switches it off before the lights go out, and measure again after each change. Light and CO₂ hours