CO2 Refill Predictor
CO2 Refill Calculator predicts your cylinder empty date from bubble rate and run time — with a conservative efficiency factor for real-world pressure drop.
Calculator
Weigh your cylinder on kitchen scales and subtract the tare weight marked on the body
Count bubbles in your diffuser over 10 seconds and divide by 10
Typically matches your lighting period
Enter your values above to see results
Keep your CO2 topped up
Pressurised kits and disposable cartridge options for all tank sizes.
Complete pressurised CO2 system — includes regulator, diffuser, and cylinder for a ready-to-run setup
Citric acid and baking soda refills for DIY generators — lower cost than pressurised, good for smaller tanks
Disposable cartridges for Fluval systems — convenient for nano tanks and travel
I’ve used everything listed in my own tanks. If you buy through a link, I earn a small commission — at no extra cost to you, and it never affects what I recommend.
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Frequently Asked Questions
- How long does a 500g CO2 cylinder last in an aquarium?
- A 500g cylinder contains roughly 253,000 mL of CO2. At a typical rate of 1–2 bubbles per second running 8 hours a day, you can expect it to last 55–110 days theoretically, or 40–80 days in practice after accounting for end-of-tank pressure drop and regulator inefficiency. Actual duration varies considerably with diffuser type, needle valve setting, and tank size. The calculator above gives you a personalised estimate based on your exact setup.
- How do I count bubbles per second accurately?
- The simplest method is to count bubbles from your diffuser over exactly 10 seconds and divide by 10. For example, 15 bubbles in 10 seconds = 1.5 BPS. Use a stopwatch for accuracy — a casual count is often significantly off. If you have a bubble counter (a small inline device many regulators include), the count is easier to read directly. Note that at very high bubble rates (above 3–4 BPS), individual bubbles may merge and are harder to count individually; an inline flow meter or drop checker gives more reliable data at those rates.
- Why does my CO2 run out faster than expected?
- Several factors cause real-world CO2 consumption to exceed theoretical calculations. The most significant is end-of-tank pressure drop: as the cylinder empties, pressure falls, but single-stage regulators often respond by increasing flow rather than maintaining it — sometimes dramatically, causing a 'dump' that empties the remaining gas rapidly. Diffuser back-pressure, micro-leaks at fittings, and CO2 dissolving into airline tubing also contribute. This calculator applies a 0.75 efficiency factor (25% reduction from theoretical) as a conservative baseline, but a dual-stage regulator can significantly improve real-world accuracy.
- What is an end of tank dump?
- An end of tank dump (EOT dump) happens when a CO2 cylinder nears empty and the internal pressure drops below the regulator's operating range. With a single-stage regulator, the working pressure rises rather than falls as inlet pressure drops — causing the regulator to suddenly increase CO2 flow dramatically. This can flood the tank with CO2, dropping pH to dangerous levels and potentially killing fish within hours. Symptoms include all fish at the surface gasping, a sudden pH crash, or unusually rapid CO2 consumption at the end of a cylinder. A dual-stage regulator prevents this by adding a second pressure reduction stage that maintains stable output even as cylinder pressure falls.
- Should I get a larger CO2 cylinder?
- Almost always yes, if you have the space. The cost per gram of CO2 drops significantly as cylinder size increases — a 2kg cylinder typically costs 30–50% less per gram than a 500g cylinder when you include refill costs. A larger cylinder also means fewer refills per year, more stable pressure over the cylinder's life, and a longer warning period before running out. The main considerations are size and weight — a 5kg cylinder is heavy and takes up space under the aquarium stand. For most 60–200 litre planted tanks, a 2kg cylinder strikes the best balance of cost, duration, and practicality.