Guide
pH and CO2 in the aquarium
You can work out the CO2 concentration in your water from two tests: pH and KH. Below are the formula, a ready chart and tips on keeping CO2 good for plants and safe for fish.
CO2 formula from pH and KH
CO2 [mg/l] = 3 × KH [°dKH] × 10(7 − pH)
The formula assumes that KH comes only from carbonates. Peat, humic acids and buffered fertilisers inflate the result, so treat it as an estimate and compare it with a drop checker. 1 °dKH equals about 17.9 ppm (mg/l) of CaCO3.
CO2 chart (mg/l) by KH and pH
| KH \ pH | 6.4 | 6.6 | 6.8 | 7.0 | 7.2 |
|---|---|---|---|---|---|
| 3 °dKH | 36 | 23 | 14 | 9 | 6 |
| 4 °dKH | 48 | 30 | 19 | 12 | 8 |
| 5 °dKH | 60 | 38 | 24 | 15 | 9 |
| 6 °dKH | 72 | 45 | 29 | 18 | 11 |
| 8 °dKH | 96 | 60 | 38 | 24 | 15 |
What CO2 level is right
- below 15 mg/l: too little for demanding plants, algae get the upper hand,
- 15–35 mg/l: the working range for a planted tank,
- above 35 mg/l: fish gasp at the surface; reduce the dose.
How to automate it
The aquarium monitor measures pH with a probe, knows your KH from your tests and calculates CO2 continuously. The CO2 plug switches off by itself when pH drops below the threshold, and the chart shows whether CO2 rises in time before the lights come on.
Frequently asked questions
How much should pH drop after turning on CO2?
A drop of about 1.0 pH compared with degassed water corresponds to roughly 30 mg/l of CO2. A drop of 0.5 is about 10 mg/l.
Is pH 6.4 harmful to fish?
The value itself usually is not, but with high KH it means a lot of CO2. What matters is the CO2 concentration, not pH alone. Check it in the chart above.
Keep an eye on your tank from your phone
The app is free and ad-free. The monitor runs on your Raspberry Pi.