Fish care calculator

Aquarium CO2 Cylinder Duration Tool | FishZone

Estimate how many days or months your pressurized CO2 cylinder will last before it needs a refill. Enter your cylinder size, bubbles per second, and daily photoperiod to get an accurate duration — no guesswork required.

  • Scientific Formula
  • Reviewed by Experts
  • Updated May 2026
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Risk Assessment

Inputs

3

Difficulty

Beginner

Calculation Time

Instant

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Quick Answer

A standard 5 lb CO2 cylinder holds approximately 1,200 liters of expanded gas. At 2 bubbles per second over an 8-hour photoperiod, it lasts around 167 days. At 4 BPS over 10 hours, the same tank runs out in roughly 67 days. This calculator does the math for you — enter your cylinder size, injection rate (BPS), and daily on-time to see your exact estimated duration in days and months.

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Enter the labeled values below. Results appear without leaving this page.

How to Use This Calculator (Worked Example)

This tool requires 3 key inputs:

  1. Bubbles Per Second (BPS): Enter your specific value (e.g., from your tank's test kit or dimensions).
  2. Daily Photoperiod (Hours On): Enter your specific value (e.g., from your tank's test kit or dimensions).
  3. Cylinder Size: Enter your specific value (e.g., from your tank's test kit or dimensions).

Example: If you input average baseline values, the calculator will immediately process the formula and return the recommended output and safety warnings above.

Method

This tool uses the visible inputs on the page and returns practical aquatics guidance for maintaining your tank.

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The Problem

Hobbyists running pressurised CO2 systems have no reliable way to know when their cylinder will run empty without tracking daily usage manually. Running out unexpectedly causes CO2 crashes that trigger algae blooms, and cylinders running dry with single-stage regulators can cause lethal End-of-Tank Dump events. This calculator eliminates the guesswork and enables proactive refill planning.

When to Use This Tool

Use this calculator when first setting up a CO2 system to establish a refill schedule, whenever you adjust your BPS or photoperiod, when comparing the cost-effectiveness of different cylinder sizes, or any time you want to know how many days of gas remain at your current injection settings.

Who is this for?

  • Freshwater planted aquarium hobbyists using pressurised CO2 injection
  • Aquascapers running high-tech setups with Dutch, Nature Aquarium, or Iwagumi styles
  • Beginners setting up their first CO2 system who want to plan ahead
  • Intermediate hobbyists looking to reduce gas waste and cut refill costs
  • Advanced planted tank keepers managing multiple tanks on a shared CO2 supply
  • Fish keepers using sensitive species concerned about End-of-Tank Dump events

High-tech planted tanks rely on a constant, stable supply of pressurized carbon dioxide to fuel explosive plant growth and outcompete algae. Running out of CO2 unexpectedly causes dissolved CO2 levels to crash overnight — often triggering a devastating algae bloom, including Black Beard Algae (BBA), within 48 to 72 hours. This calculator exists to prevent exactly that.

Who Should Use This Calculator?

This tool is designed for any freshwater aquarium hobbyist running a pressurized CO2 injection system — from beginners setting up their first planted tank to experienced aquascapers managing multiple high-tech setups. If you use a CO2 cylinder and a regulator (whether a single-stage budget regulator or a high-end dual-stage unit), this calculator will tell you exactly when to schedule your next refill.

It is especially valuable for hobbyists who: run a densely planted aquascape relying on stable CO2 for plant health; use a solenoid on a timer to conserve gas during dark hours; keep sensitive fish species that cannot tolerate sudden swings in water chemistry caused by an End-of-Tank Dump; or simply want to budget and plan refill trips efficiently.

The Math Behind the Bubble

Most regulators use a visual Bubble Counter to measure injection rate. While bubble size can vary slightly depending on the counter fluid (water versus mineral oil), a standard aquarium CO2 bubble is approximately 0.125 milliliters (125 microliters) at atmospheric pressure.

A standard 5 lb aluminum CO2 cylinder holds roughly 1,200 liters (or 1,200,000 milliliters) of expanded CO2 gas when fully depressurized to atmospheric pressure. A 10 lb cylinder holds approximately 2,400 liters. If you run your system at 3 Bubbles Per Second (BPS) for an 8-hour photoperiod, you are injecting roughly 10,800 milliliters (10.8 liters) of gas per day. Dividing 1,200,000 mL by 10,800 mL gives approximately 111 days — about 3.5 months — from a single 5 lb fill.

The formula this calculator uses is: Duration (days) = Cylinder Volume (mL) ÷ (BPS × 0.125 mL × Photoperiod Hours × 3600 seconds). The result is then converted to weeks and months for easy planning.

Why Regulators Matter — The End-of-Tank Dump

As a CO2 cylinder empties, the liquid CO2 inside gradually flashes into gas. While liquid CO2 remains inside the cylinder, the internal pressure stays relatively constant (around 800–900 psi). Once all the liquid is consumed, the cylinder becomes a pure gas tank and internal pressure begins to drop rapidly.

On inexpensive single-stage regulators, this sudden pressure drop paradoxically causes the working (outlet) pressure to spike upward — sending a massive, uncontrolled dose of CO2 into the aquarium. This is called the End-of-Tank Dump, and it can kill an entire tank of fish within hours. A quality dual-stage regulator compensates for this drop and prevents the spike. However, knowing exactly when your cylinder will run empty — and replacing it before it reaches that point — is your most reliable defense, regardless of regulator quality.

Common Problems This Calculator Solves

Many hobbyists either over-inject CO2 (wasting gas and risking fish health) or under-inject (resulting in algae outbreaks and poor plant growth). This tool helps you understand how your daily injection decisions translate directly into cylinder lifespan, empowering you to find a sustainable balance. It also helps beginners understand that a higher BPS rate is not always better — it burns through your cylinder faster and can stress fish if CO2 levels exceed 30 ppm.

When to Use This Calculator

Use it when you first set up a pressurized CO2 system to know when to expect your first refill. Use it again whenever you adjust your BPS or photoperiod duration. Use it to compare the cost-effectiveness of a 5 lb versus 10 lb cylinder for your specific setup. And use it any time you want to schedule a refill before your tank runs dry — rather than discovering it empty on a Monday morning.

Reference Table

Static Aquarium CO2 Cylinder Duration Estimator Reference

These baseline ratios and decision rules are included directly in the page so visitors can review core guidance before using the interactive calculator.

Scenario Baseline Rule Safety Note
Inputs used by this tool Bubbles Per Second (BPS), Daily Photoperiod (Hours On), Cylinder Size The interactive calculator refines the result from these inputs.
Outputs generated Days, Months The static table gives baseline logic; final value depends on entered values.
Cylinder SizeApprox. Expanded VolumeEst. Duration at 2 BPS / 8hrEst. Duration at 3 BPS / 8hrEst. Duration at 4 BPS / 10hrTypical Refill Cost (USD)
5 lb~1,200 L~167 days~111 days~50 days$15–$25
10 lb~2,400 L~333 days~222 days~100 days$20–$35
20 lb~4,800 L~667 days~444 days~200 days$30–$50
Troubleshooting Guide
1 CO2 ran out much sooner than the calculator estimated

2 Fish gasping at the surface every morning

3 Drop Checker stays blue despite running CO2

Glossary of Terms
BPS (Bubbles Per Second)
The visual rate of CO2 injection measured by counting bubbles passing through a bubble counter. One standard aquarium bubble is approximately 0.125 mL at atmospheric pressure.
Drop Checker
A glass or acrylic device that hangs inside the aquarium and uses 4dKH reference solution with bromo blue dye to indicate dissolved CO2 levels. Green indicates ~30 ppm (ideal); blue is too low; yellow is dangerously high.
End-of-Tank Dump
A dangerous event that occurs with single-stage regulators as a CO2 cylinder empties. The sudden internal pressure drop causes the outlet pressure to spike, flooding the aquarium with a lethal CO2 dose.
Dual-Stage Regulator
A CO2 regulator that uses two pressure-reduction stages, providing stable outlet pressure even as cylinder pressure drops. Strongly recommended for inhabited aquariums to prevent End-of-Tank Dump events.
Solenoid
An electronically controlled valve fitted to a CO2 regulator that opens and closes on a timer. It allows the CO2 system to run only during the photoperiod, preventing overnight CO2 buildup and conserving cylinder gas.
Photoperiod
The number of hours per day that the aquarium lights are on. CO2 injection should run only during the photoperiod, as plants only consume CO2 during photosynthesis when light is present.
ppm (Parts Per Million)
The unit used to measure dissolved CO2 concentration in aquarium water. The optimal range for planted tanks is 25–30 ppm. Above 35 ppm is stressful or lethal to fish.
4dKH Reference Solution
A precisely calibrated carbonate hardness solution used inside a Drop Checker. Its fixed hardness allows the indicator dye to respond only to CO2 concentration changes, giving an accurate colour reading.
Bubble Counter
A small transparent device filled with water or mineral oil through which CO2 passes before entering the aquarium. It allows the hobbyist to visually count and adjust the injection rate in Bubbles Per Second.
Black Beard Algae (BBA)
A particularly stubborn, dark-coloured algae that typically appears after sudden drops in CO2 concentration. Its appearance is a strong indicator of CO2 instability or fluctuation in a planted tank.
Scientific References
  1. CO2 in Planted Aquaria — The Estimative Index
  2. Understanding CO2 Phase Changes and End-of-Tank Dumps

Frequently Asked Questions

How long will a 5 lb CO2 cylinder last in a planted aquarium?
It depends entirely on your injection rate and daily photoperiod. At a moderate 2 BPS over an 8-hour photoperiod, a 5 lb cylinder typically lasts around 167 days (roughly 5.5 months). At a heavier 4 BPS over a 10-hour photoperiod, the same tank runs out in about 67 days. Use the calculator above with your exact settings to get a personalized estimate.
How long will a 10 lb CO2 cylinder last?
A 10 lb cylinder holds approximately twice the gas of a 5 lb — around 2,400 liters expanded. At the same 2 BPS over 8 hours, it would last roughly 333 days. For most hobbyists running moderate injection, a 10 lb cylinder is a worthwhile investment since refill cost is only slightly higher than a 5 lb but lasts nearly twice as long.
What is BPS and how do I measure it?
BPS stands for Bubbles Per Second and is the standard visual measure used with aquarium bubble counters. To measure it, count the number of bubbles that pass through your bubble counter in 10 seconds and divide by 10. Most planted tanks run between 1 BPS (small or lightly planted tanks) and 5 BPS (large, heavily planted tanks). It is a visual reference, not a precise scientific unit — bubble size can vary slightly depending on your counter fluid.
How many bubbles per second do I actually need for my tank?
There is no universal number. A 10-gallon nano tank might only need 0.5–1 BPS, while a densely planted 75-gallon high-tech tank might need 4–6 BPS. The correct rate is the one that brings your CO2 concentration to roughly 25–30 ppm during the photoperiod. The only reliable way to verify this is with a Drop Checker filled with 4dKH reference solution — you want the fluid to turn lime green. Yellow means too much CO2; blue means too little.
Should I run my CO2 at night?
No, and this is one of the most common beginner mistakes. Plants only photosynthesise and consume CO2 when the lights are on. At night they switch to respiration — consuming oxygen and releasing CO2. Running your CO2 solenoid overnight wastes gas and allows CO2 to accumulate to toxic levels, which can suffocate fish by morning. Always put your solenoid on a timer set to match your lighting schedule, and consider turning CO2 on about one hour before the lights come on to pre-charge the water.
What is an End-of-Tank Dump and is it dangerous?
Yes, it is potentially lethal. When a CO2 cylinder is nearly empty, the internal pressure drops rapidly. On cheap single-stage regulators, this drop causes a counter-intuitive spike in outlet pressure, flooding the aquarium with a massive uncontrolled CO2 dose. Fish can die within hours. Signs of an impending dump include a working pressure gauge that begins creeping up as the cylinder empties. To protect your tank, use a dual-stage regulator and — more importantly — replace your cylinder before it runs completely empty. This calculator helps you know exactly when that point is approaching.
What is the difference between a single-stage and dual-stage regulator?
A single-stage regulator reduces cylinder pressure to working pressure in one step. As the cylinder pressure drops (especially near empty), the outlet pressure can become unstable and spike, causing an End-of-Tank Dump. A dual-stage regulator uses two pressure-reduction chambers in sequence, making the outlet pressure far more stable even as the cylinder empties. For fish safety, a dual-stage regulator is the recommended choice for any inhabited aquarium.
How accurate is this CO2 cylinder duration calculator?
The calculator uses a well-established formula based on the standard bubble volume of approximately 0.125 mL and published cylinder capacities for standard aluminum CO2 tanks. Real-world results typically fall within 10–15% of the estimate, depending on small variations in your specific bubble counter, the brand and age of your cylinder, and whether you use a solenoid consistently. Treat the result as a reliable planning guide, not a precise countdown timer.
Where can I refill my CO2 cylinder?
The cheapest and most common option is a local welding supply company such as Airgas, Praxair, or your nearest independent welding shop. Fire extinguisher service companies also handle CO2. Homebrewing supply shops are another excellent option and often welcome aquarium hobbyists. A refill for a 5 lb cylinder typically costs USD $15–$25, and many shops offer a cylinder swap programme so you get a freshly filled tank immediately rather than waiting. Avoid CO2 cartridges sold for sodastream-type devices — they are far more expensive per gram of CO2.
Can I use this calculator for a nano or mini CO2 system?
This calculator is primarily optimised for standard pressurised cylinders in the 5 lb and 10 lb range used with full regulators. If you are using a nano CO2 system with small proprietary cartridges (such as the Fluval or JBL compact systems), the same formula applies — you just need to know the total gas volume of your cartridge in liters or milliliters, which is usually listed on the manufacturer's specifications.
What happens to my plants if I run out of CO2 suddenly?
When CO2 injection stops abruptly, dissolved CO2 in the water quickly depletes. Fast-growing stem plants will begin to slow visibly within 24–48 hours. More critically, the biological balance of your tank shifts — algae that previously lost the competition for CO2 and nutrients can seize the opportunity to bloom rapidly. Black Beard Algae (BBA), Green Spot Algae (GSA), and hair algae are the most common post-crash invaders. Consistent CO2 is one of the three pillars of a successful high-tech planted tank, alongside light and nutrients.
Is higher BPS always better for plant growth?
No. More CO2 is not always better, and excess CO2 is actively harmful. The target concentration for planted tanks is 25–30 ppm. Above 30–35 ppm, fish begin to show signs of CO2 toxicity — gasping at the surface, lethargy, and loss of colour. Beyond being dangerous, a very high BPS burns through your cylinder faster and increases costs without additional plant benefit. Dial in your BPS with a Drop Checker rather than simply turning it up.
How do I know if my CO2 levels are too high for my fish?
Watch your fish carefully, especially in the first hour after your CO2 turns on each morning. Early signs of CO2 stress include fish congregating near the surface or around the filter outflow (where oxygen is highest), rapid gill movement, and general lethargy. A Drop Checker turning yellow is a strong chemical warning. If you see these signs, immediately increase surface agitation (open a lily pipe, turn up the filter), turn off the CO2, and check your BPS. Reduce your injection rate before the next photoperiod.
What is a Drop Checker and why do I need one?
A Drop Checker is a small glass or acrylic device that hangs inside your aquarium and holds a small volume of 4dKH reference solution mixed with a pH-sensitive indicator dye (bromo blue). CO2 diffuses through an air gap into the solution and changes its colour: blue indicates low CO2 (below ~15 ppm), green indicates ideal CO2 (around 25–30 ppm), and yellow indicates dangerously high CO2 (above ~35 ppm). It is an inexpensive but essential tool for any CO2-injected tank. Note that it reads with a lag of 1–2 hours, so it reflects recent average levels rather than the current instant.
Does my photoperiod length really affect cylinder duration that much?
Significantly yes. Adding just two hours to your daily photoperiod (going from 8 hours to 10 hours) increases daily gas consumption by 25%, reducing your cylinder life by the same proportion. Conversely, if you find your cylinder running out sooner than expected, reducing your photoperiod by an hour or two is a simple way to extend it without changing your plant growth substantially — many advanced aquascapers run 6–8 hour photoperiods and achieve excellent results.
Why does my CO2 cylinder feel cold to the touch?
This is completely normal and actually a sign the cylinder is working correctly. The liquid CO2 inside undergoes an endothermic phase change as it vaporises into gas, absorbing heat from the surroundings. This is why the cylinder and regulator body can feel cold or even show frost in humid environments during heavy use. It does not indicate a leak or problem with the system.
How do I check how much CO2 is left in my cylinder without a calculator?
The most reliable method is to weigh the cylinder on a kitchen scale and subtract the tare weight (the empty cylinder weight, usually stamped on the collar). The difference is the remaining CO2 by weight in pounds or kilograms. A cylinder gauge showing pressure is less helpful here because CO2 pressure stays relatively constant until almost all the liquid is gone — the pressure drop happens very suddenly near the end, which is exactly what makes an End-of-Tank Dump so dangerous.
Can I connect two CO2 cylinders together to double my duration?
Yes, experienced hobbyists do this using a manifold or a simple T-connector between two cylinders and one regulator. However, both cylinders need to be at similar pressures for this to work correctly, and you need to ensure your regulator and fittings are rated for the combined setup. A simpler solution for most hobbyists is to simply upgrade to a 10 lb or 20 lb cylinder and reduce refill frequency.
Is pressurised CO2 injection worth it compared to liquid carbon (like Excel)?
Pressurised CO2 is significantly more effective and ultimately cheaper per unit of carbon delivered for any tank above about 20 gallons. Liquid carbon products like Seachem Excel or Aquario NEO Carbon work as an algaecide and supplement but cannot achieve the CO2 concentrations that pressurised gas provides. For a serious planted aquascape — especially one with demanding stem plants, carpeting plants, or mosses — pressurised CO2 is the standard. Liquid carbon is a practical alternative for low-tech, low-light setups.
How do I use this calculator for the first time if I have never tracked my BPS?
Start by setting your regulator to a rate appropriate for your tank size — roughly 1 BPS per 10–15 gallons as a starting point. Count your bubbles for 10 seconds and divide by 10 to get your BPS. Enter that number and your planned daily photoperiod into the calculator. Run the system for a week, observe your Drop Checker colour and fish behaviour, adjust BPS as needed, and then re-run the calculator with your finalised BPS to get an accurate estimate for your setup.