A CO2 cylinder on a planted aquarium drains slowly enough that it is easy to be caught out by an empty bottle mid-week. This calculator turns your injection rate into a grams-per-day figure and tells you roughly how many days of CO2 you have left, so you can order a refill in good time.
How it works
CO2 usage is mass over time. The tool supports two input modes:
Bubble mode converts your bubble-per-second injection rate to grams per day using a standard bubble mass and your daily photoperiod on-time:
grams/day = bubbles_per_sec × 0.0001 g × 3600 × on_hours
days left = remaining_grams / grams_per_dayGrams mode accepts a measured grams-per-day figure directly — the most accurate method, since it captures your specific diffuser efficiency and bubble counter calibration. Weigh the cylinder before and after a week of normal use; the difference divided by 7 is your real daily consumption rate.
Common cylinder sizes for planted aquariums
| Cylinder size | Typical use case |
|---|---|
| 95–100 g (paintball-style) | Nano and small tanks; convenient but refills frequent |
| 500 g | Mid-size tanks; a good balance of size and duration |
| 1000 g (1 kg) | Larger tanks or high injection rates; monthly refills typical |
| 2000 g (2 kg) | Large display tanks; least frequent refills |
Worked example
A 500 g cylinder with 450 g of gas remaining, injecting 2 bubbles per second for 9 hours a day:
2 × 0.0001 × 3600 × 9 = 6.5 g/day
450 ÷ 6.5 ≈ 69 days
That is roughly 10 weeks — more than enough to order a refill well in advance.
Tips for accuracy and safety
- Run CO2 only during the photoperiod. Use a solenoid valve and a timer to open CO2 flow about an hour before lights-on and close it an hour before lights-off. Plants only photosynthesise (and consume CO2) under light; running it overnight wastes gas and causes pH to drop dangerously low.
- Order your refill when the estimate drops below 10 days, not when the cylinder is nearly empty. Regulators lose stable pressure when a cylinder is almost exhausted, causing injection rates to drift before the gas actually runs out.
- Bubble size varies. Different diffusers produce different bubble sizes, and the standard 0.0001 g per bubble is an approximation. The grams-mode input (measured by weighing the cylinder) gives a much more accurate duration for your specific setup.
- Track your refill dates. Knowing how long a full cylinder lasts in your setup lets you predict the next refill and stock a spare without uncertainty.