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CO₂ calculator for aquariums
Enter KH and pH to see how much CO₂ is in the water, which pH matches your target and roughly how long your bottle will last.
How it is calculated
CO₂, carbonate hardness and pH are tied together. That gives the formula used in the hobby: CO₂ (mg/L) = 3 × KH × 10(7 − pH). At KH 5 and pH 6.9 that is about 19 mg/L.
The other way round you get the pH that matches your target CO₂ at your KH: target pH = 7 − log10(CO₂ ÷ (3 × KH)). That is the setpoint for a pH controller.
Limits: the formula assumes only carbonates buffer the pH. Peat, catappa leaves, soil or phosphate buffers shift the pH without changing CO₂, and the calculator then shows too much CO₂. A drop checker shows the real value as a colour.
How much CO₂ is right?
- Below 10 mg/L: normal without a CO₂ system, too little for demanding plants.
- 15 to 25 mg/L: the usual range for planted tanks. Densely planted tanks and aquascapes rather 25 mg/L.
- Up to 30 mg/L: upper limit for most fish.
- With shrimp, crayfish or sensitive fish: 20 mg/L at most.
Always raise CO₂ slowly over several days and watch your animals in the morning when the lights come on. Switch CO₂ off at night (solenoid on a timer) or move the surface.
More than a calculator: CO₂ in the app
In Aqualizer the CO₂ calculator uses the plants and animals that are actually in your tank. The app remembers every bottle change, tells you when the bottle is likely to run out and warns you when your CO₂ is unusually low.
- Target CO₂ from your livestock
- Bottle history and spare bottles
- Heads-up before the bottle runs out
Coming soon to Google Play – try it now
Join the beta on Android
CO₂ calculator FAQ
How do I calculate the CO₂ level in my aquarium?
From KH and pH: CO₂ in mg/L = 3 × KH × 10 to the power of (7 − pH). At KH 5 and pH 6.9 that is about 19 mg/L. The calculator above does this for you.
What CO₂ level is ideal in an aquarium?
Planted tanks usually run at 15 to 25 mg/L, 30 mg/L is the upper limit for most fish. With shrimp, crayfish or sensitive fish, stay at 20 mg/L or below.
Why is the calculated CO₂ sometimes wrong?
The formula assumes only carbonates determine the pH. Humic substances from peat or leaves, soil and phosphate shift the pH, so too much CO₂ is calculated. A drop checker shows the actual value.
How long does a CO₂ bottle last?
Mostly a matter of bubble rate and bubble size. A 500 g bottle at one medium bubble per second for 8 hours a day lasts about three months. Outgassing and leaks shorten that, which is why the app uses your real bottle changes.
Which pH should I set on my CO₂ controller?
The target pH from the calculator, which follows from your KH and the CO₂ you want. If the KH changes, for example after a water change, adjust the setpoint too.