The short answer
Pick your CO2 cylinder to match your tank size and how often you want to refill. Broadly, there are two routes: small disposable canisters, cheap and easy but quickly emptied — fine for nanos or trying CO2 out — and larger refillable cylinders, which cost more upfront but are far cheaper to run and last months on a typical planted tank. For anything beyond a small tank, refillable wins on cost.
Disposable vs refillable
- Disposable canisters — screw-on, self-contained and simple. Low entry cost and no need to visit a gas supplier. The downside: they hold little gas, run out fast, and cost much more per gram of CO2. Best for nano tanks or testing the water before committing.
- Refillable cylinders — larger steel or aluminium bottles you refill or exchange at a gas or welding supplier. Higher initial outlay, but the running cost per refill is a fraction of disposables, and a single fill lasts a long time. The sensible choice for medium and large tanks.
Sizing it to your tank
The right size balances how much CO2 you inject (higher bubble rates on bigger, brighter tanks use more) against how often you’re willing to refill.
- Nano / small tanks — a small disposable or a compact refillable is plenty.
- Medium tanks — a mid-size refillable cylinder gives months between fills.
- Large or high-light tanks — a bigger cylinder means fewer refills and better value over time.
Buying a larger refillable than you strictly need is usually smart: the gas costs the same, and you refill far less often.
Putting it together
Whatever size you pick, the cylinder feeds a regulator, then tubing to a check valve and diffuser. See how to connect a CO2 regulator and CO2 for beginners. For complete kits, browse the CO2 systems hub and our best CO2 system picks.