CO₂ / pH / KH chart for planted aquariums
Estimate the dissolved CO₂ level in your planted tank from pH and KH readings. Enter your values below — the chart highlights your zone and shows whether you're in the optimal range for plant growth.
From a liquid reagent or electronic pH meter
From a KH drop test or carbonate hardness meter
Estimated CO₂
Calculated from: CO₂ [mg/L] = 3 × KH × 10^(7 − pH). Valid when the carbonate system is the dominant buffer.
CO2 concentration (mg/l)
CO2 level based on pH and KH. Green = recommended range.
| KH 2 | KH 4 | KH 6 | KH 8 | KH 10 | KH 12 | KH 14 | KH 16 | KH 18 | KH 20 | |
|---|---|---|---|---|---|---|---|---|---|---|
| pH 7,8 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 |
| pH 7,6 | 2 | 3 | 5 | 6 | 8 | 9 | 11 | 12 | 14 | 15 |
| pH 7,4 | 2 | 5 | 7 | 10 | 12 | 14 | 17 | 19 | 21 | 24 |
| pH 7,3 | 3 | 6 | 9 | 12 | 15 | 18 | 21 | 24 | 27 | 30 |
| pH 7,2 | 4 | 8 | 11 | 15 | 19 | 23 | 27 | 30 | 34 | 38 |
| pH 7,1 | 5 | 10 | 14 | 19 | 24 | 29 | 33 | 38 | 43 | 48 |
| pH 7,0 | 6 | 12 | 18 | 24 | 30 | 36 | 42 | 48 | 54 | 60 |
| pH 6,9 | 8 | 15 | 23 | 30 | 38 | 45 | 53 | 60 | 68 | 76 |
| pH 6,8 | 10 | 19 | 29 | 38 | 48 | 57 | 67 | 76 | 86 | 95 |
| pH 6,7 | 12 | 24 | 36 | 48 | 60 | 72 | 84 | 96 | 108 | 120 |
| pH 6,6 | 15 | 30 | 45 | 60 | 75 | 90 | 105 | 121 | 136 | 151 |
| pH 6,4 | 24 | 48 | 72 | 96 | 119 | 143 | 167 | 191 | 215 | 239 |
| pH 6,2 | 38 | 76 | 114 | 151 | 189 | 227 | 265 | 303 | 341 | 379 |
Save these values in AquaPilot
Track pH, KH and CO₂ over time — the app builds trends and alerts you if your CO₂ drifts out of the optimal range.
How it works
- 1
Measure pH and KH
Use a drop test, liquid reagent or electronic meter. Measure at the same time of day — CO₂ shifts pH throughout the photoperiod.
- 2
Enter the values
Type your pH and KH into the form above. The chart instantly highlights the cell matching your reading.
- 3
Read the CO₂ level
The result in mg/L tells you whether you're low (under 5), in the plant-friendly zone (15–30), or dangerously high (over 45) for fish.
When to use it
Setting up CO₂ injection
Tune your bubble rate to land between 20–35 mg/L — enough for lush plant growth without stressing fish.
Diagnosing plant problems
Stunted growth or algae outbreaks often trace back to unstable CO₂. A pH/KH reading tells you the real level, not the one on the solenoid.
Switching to a low-tech tank
Without injection you should expect 3–5 mg/L from surface exchange — use the chart to confirm and pick plants that tolerate it.
Understanding the pH–KH–CO₂ relationship
In a freshwater aquarium, dissolved CO₂ forms carbonic acid that lowers pH. The stronger the carbonate buffer (KH), the more CO₂ it takes to shift pH down by the same amount — which is why the chart needs both values to estimate CO₂.
The formula
The standard approximation used by the chart. It assumes the carbonate/bicarbonate system is the main buffer in the water — true for most tap-water aquariums.
CO₂ zones
Common targets for planted tanks. Below 5 mg/L plants starve; above 45 mg/L fish start gasping at the surface.
- Too low< 5 mg/L
- Good5–25 mg/L
- Optimal25–45 mg/L
- Risky> 45 mg/L
Where the chart can mislead you
The formula works well for plain tap-water aquariums. It stops being accurate when something other than the carbonate system is driving pH.
Phosphate buffers
Commercial pH-down or phosphate buffers (common in discus and African-cichlid tanks) throw off the estimate — pH drops without CO₂ rising.
Driftwood, peat and tannins
Humic and tannic acids from driftwood or peat also lower pH. The chart will overestimate CO₂ in blackwater-style setups.
Measurement errors
A 0.2 pH error roughly doubles or halves the CO₂ estimate. Calibrate your meter or cross-check with a drop checker.
Worked examples
A few typical combinations and what they mean for a planted tank.
| pH | KH | CO₂ | What it means |
|---|---|---|---|
| 7.2 | 3 | 5 mg/L | Too low — plants struggle |
| 7.0 | 5 | 15 mg/L | Good for mid-light plants |
| 6.8 | 6 | 29 mg/L | Optimal for high-tech tanks |
| 6.6 | 8 | 60 mg/L | Risky — reduce injection |
Frequently asked questions
Is the estimate accurate?
For clean tap-water aquariums, yes — typically within ±20%. It becomes unreliable when pH is driven by phosphate buffers, peat, driftwood tannins, or other non-carbonate acids.
What CO₂ level should I target?
For most planted tanks, 20–30 mg/L during the photoperiod is ideal. Drop checkers turn lime-green around 30 mg/L. Below 10 mg/L plants struggle; above 45 mg/L fish may gasp.
Why does my pH drop after lights on?
CO₂ injection starts before the lights in most setups, so CO₂ accumulates and pH drops 0.8–1.2 units. This is expected and safe if you're in the optimal zone.
Can I use this for tap water before a water change?
You can, but tap water is often supersaturated or degassed at the source — pH/KH readings stabilize after a few hours. The chart is most accurate on settled tank water.
Is this chart specific to any test kit?
No. The formula is universal — any accurate pH and KH reading (JBL, Salifert, API, electronic) works. Accuracy of the estimate depends on the accuracy of your test.
Related tools
Want AquaPilot to do this automatically?
Log pH, KH and CO₂ regularly, and the app will spot drifts and suggest fixes before plants or fish suffer.
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