Free · No login required

CO2 Calculator
for Aquariums

Calculate CO2 level from pH + KH, dial in your bubble rate and estimate cylinder duration – right here, no sign-up needed.

Inputs
CO2 status
Bubble rate
typical 0.5 – 4 bps
default 0.10 ml
Cylinder duration
300 g · 500 g · 1000 g · 2000 g
Results
Enter pH + KH for a CO2 estimate.
0101530≥35
Enter KH + pH for the target pH.
Enter bps for flow rate.
Timer: turn CO2 on 30 min before lights on, off 60 min before lights out.
Help – understanding CO2, pH and KH

Formula: CO2 [mg/L] ≈ 3 × KH [dKH] × 10(7 – pH)

Situation mg/L Note
Too low < 10 Plants stunted, algae
Acceptable 10–15 Lightly planted tanks
Optimal 15–30 Good fish/plant compromise
Borderline 30–35 Only with O2 monitoring
Dangerous > 35 Stress / suffocation
Shrimp / crayfish max. 20 Sensitive livestock

Counting bubbles: count for 10 seconds, divide by 10. 15 bubbles in 10 s = 1.5 bps.

Drop checker: Blue = too little · Green = optimal · Yellow = too much.

Increase slowly: Never raise more than 0.5 bps at a time, then wait 24 h and re-check pH. CO2 dissolves slowly.

With a free account
The calculator gets to know your tank.

Right now it works with whatever you type in. With an account, pH and KH are pre-filled from your latest measurement automatically – and your livestock is taken into account.

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Livestock-aware limits

Got shrimp in your tank? The calculator automatically caps the limit at 20 mg/L – no need to remember it yourself.

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Cylinder tracking

Log every swap. After 2–3 cylinders you'll see your personal average runtime and when the next one is due.

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pH + KH pre-filled

The latest readings from your tank are pulled into the calculator automatically.

Sign up for free → Already have an account
Frequently asked questions
How do I calculate CO2 from pH and KH?
Formula: CO2 [mg/L] ≈ 3 × KH [dKH] × 10^(7 – pH). Example: KH 6, pH 7.0 → 3 × 6 × 1 = 18 mg/L. At KH 6 and pH 6.8 → 3 × 6 × 10^0.2 ≈ 28 mg/L. The calculator above does this automatically.
How much CO2 does my aquarium need?
15–30 mg/L for a typical fish tank. Heavily planted high-tech tanks tolerate 20–30 mg/L. For shrimp and crayfish, keep it at 20 mg/L max. Below 10 mg/L, plants stunt and algae take over.
How many bubbles per second is right?
Starting point: tank volume ÷ 80 as a bps guideline. What matters more than the bubble rate is the result: check pH after 24 h, then increase gradually. Never change more than 0.5 bps at a time.
How long does a CO2 cylinder last?
Formula: cylinder (g) × 1000 ÷ CO2 input (mg/min) ÷ 60 ÷ h/day. Example: 500 g, 20 mg/min, 8 h/day → 52 days. Just enter cylinder size and runtime in the calculator above.
What do I do if CO2 is too high?
Reduce the bubble rate immediately and turn on an air stone – CO2 gets driven off actively and pH rises fast. Going forward: increase more slowly and allow more time between adjustments.
Is CO2 dangerous for shrimp?
Yes. Shrimp are more sensitive than fish, so keep it at 20 mg/L max. A night shutoff is essential: without photosynthesis, CO2 keeps climbing. Set the timer so CO2 shuts off 60 min before lights out.
01 · CO2 bubble calculator

Enter pH + KH –
read the CO2 level instantly.

  • A traffic-light indicator shows: optimal, borderline or too high
  • Calculate target pH – what pH do you need for 20 mg/L CO2
  • Bubble rate recommendation matched to volume, planting and livestock
  • Calculate ramp-up time: how long until the tank reaches target
  • For shrimp / crayfish: limit automatically capped at 20 mg/L
Calculate now ↑
CO2 bubble calculator with traffic-light indicator and recommendation
02 · CO2 cylinder tracking
CO2 cylinder tracking with history and ETA

Never caught off guard
by an empty cylinder.

  • Log every cylinder swap – history and runtime are tracked
  • Average runtime from your real data – no guessing, no math
  • ETA for the next swap with a progress bar
  • Compare different cylinder sizes (500 g, 1 kg, 2 kg …)
Sign up for free →
More features

Everything CO2-related in one tool

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KH buffer warning

Too little KH? The calculator warns you before pH can crash uncontrollably – with a recommendation on what to do.

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pH controller setpoint

Using a pH controller? The calculator works out the optimal setpoint and logs it for you.

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Connected RO calculator

KH too high? Jump straight to the RO water calculator for the exact mix ratio with your tap water.

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Livestock pH match

Does your target pH fit your livestock's pH tolerance range? If not, the calculator shows what to do.

In 3 steps

Dial in CO2 the right way

1
Start low and measure

Start at 0.5–1 bps. Wait 24–48 hours, then measure pH and KH and enter them above. The CO2 value appears instantly with a traffic-light rating.

2
Increase the bubble rate in small steps

Only increase by 0.2–0.5 bps at a time, then wait another 24 hours. CO2 dissolves slowly – patience pays off and protects your livestock.

3
Let Aqualizer track your cylinder swaps

Log every swap in Aqualizer. After 2–3 cylinders the tool shows your personal average runtime and the next swap date.

Why the calculator helps

Gut feeling vs. real numbers

Without a calculator
✗Bubble rate set by feel
✗CO2 level unknown despite a pH reading
✗Cylinder runs dry – no warning
✗KH too low noticed too late
✗Shrimp under stress
With Aqualizer
✓Recommendation tailored to your tank
✓CO2 mg/L calculated instantly from pH + KH
✓ETA + history for every cylinder swap
✓KH buffer warning with a clear action
✓Automatically lower limit for shrimp
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Connect the calculator to your tank

Sign up for free – pH and KH get pre-filled automatically, the shrimp limit is set automatically, and cylinder swaps are tracked.

Sign up for free now →
Free · No credit card · No download