Buy a CO2 Cylinder for Your Planted Tank: A Buyer's Guide

You buy a CO2 cylinder locally rather than online, because a filled bottle is pressurised goods that most carriers will not post. Hobbyists get them from welding and industrial gas suppliers, homebrew and beverage shops, fire-extinguisher services and some aquascaping stores. Pick a refillable cylinder in a size you can lift, check that its valve thread matches your regulator, and find out who refills or exchanges it near you before you pay.

Why the cylinder is the one part you buy locally

Our complete CO2 systems say it plainly on the page: everything you need to start CO2 on an aquarium with an external filter is in the box, except the cylinder. That is not an oversight. A filled cylinder is a pressure vessel, dangerous goods for most couriers, and heavy enough to make the postage cost more than the gas inside it. The equipment travels well; the bottle does not.

Buying locally also works in your favour. A filler in your own town means a quick swap when the bottle runs dry mid-week instead of a wait on a courier, and a counter where someone can look at your regulator and tell you what fits it. Across much of Europe the same depots serve welders, brewers, cafes and aquarists from the same racks, so the gas you want is usually already on the shelf.

Refillable cylinder or disposable bottle?

A refillable cylinder, steel or aluminium, bought outright or taken on deposit, is what most planted tanks run on. You pay more at the start and much less per fill, and the bottle keeps working for years as long as it passes its periodic test. This is the route to choose if the tank is a permanent display.

The other route is small bottles that are swapped or thrown away: a SodaStream bottle, a paintball cylinder, or disposable M10x1 cartridges. None of them has a standard cylinder valve, so they run through our CO2 adapter for SodaStream and paintball bottles rather than a regulator thread. They are right for a nano aquarium, a flat with nowhere to stand a bottle, or the weeks while your main cylinder is away for a refill. On a larger tank they empty quickly and the gas costs the most per fill of any route.

• Own the bottle: the cheapest gas over time, and the testing is your errand.
• Deposit or exchange bottle: the supplier owns, tests and fills it, and you swap empties over the counter.
• SodaStream or paintball: no depot needed, small capacity, needs our adapter.
• Disposable cartridges: travel, a quarantine tank or an emergency, not a plan.

Where to buy a CO2 cylinder

There is no single aquarium channel for gas. The bottle comes from whichever local trade already handles carbon dioxide, and in most towns that is more than one of the following.

Welding and industrial gas suppliers

The default and usually the cheapest. They sell bottles outright and on rental, and they refill or exchange them while you wait. Ask whether the gas comes from their food-grade line or the industrial one; both are sold, and the counter staff will tell you which they fill from. We always recommend choosing food-grade CO2, regardless of the supplier

Homebrew, beverage and catering suppliers

Beer gas is the same carbon dioxide, and these suppliers are used to walk-in customers with a single bottle. They tend to stock the smaller sizes that fit under an aquarium cabinet, and they are the easiest place to ask about an exchange scheme.

Fire-extinguisher and safety companies

They recharge CO2 extinguishers, so they have the gas and the equipment, and many will fill an aquarium cylinder as a favour trade. They are also the people who can put a bottle through its periodic pressure test when the stamp runs out.

Aquascaping shops and second-hand listings

Some aquarium shops sell cylinders already matched to hobby regulators, which removes the thread guesswork. Second-hand bottles are fine value if the test date and the valve check out, but never buy one whose contents nobody can name.

What size CO2 cylinder should you buy?

The right size is the largest one you can lift, store upright and get refilled without a trip across the country. A bigger bottle costs less per fill and runs for months between swaps; a smaller one hides in a cabinet and is no trouble to carry to the depot. Sizes are sold either by water capacity or by the weight of gas the bottle holds, and the ranges on offer differ by market.

Two practical points. If the tank is your main display, buy one size up from what you think you need, because the injection rate goes up as the plants fill in and the demand never falls. And if you would rather not run a planted tank without gas for a few days, keep a small second bottle or the adapter route as cover while the main cylinder is being filled.

Check the valve thread before you pay

A cylinder valve is a thread standard, not a universal fitting, and this is where most mail-order mistakes happen. Our CO2 regulators are built in two versions: CGA320 for North and Latin America, and BS341 / DIN477 / W21.8x14 for the UK, Europe, Japan and Taiwan. Buy the bottle and the regulator to the same standard and the two screw together with a washer in between.

If you already own the regulator, read the version off your order or photograph its inlet and show it at the counter. If you already own the bottle, match the regulator to it instead. Adapters exist for the small-bottle routes, but an adapter is a conversion for SodaStream, paintball and cartridge bottles, not a fix for a main cylinder with the wrong thread.

Buy spare sealing washers at the same time. A replacement regulator washer costs almost nothing, and a tired one is the most common cause of the slow leak that empties a cylinder over a weekend. Fit a fresh washer every time the regulator comes off the bottle.

Buying second-hand or taking a rented bottle: what to inspect

A used cylinder is a sound buy if you know what to look at, and the whole inspection takes a minute at the seller's door.

• The test stamps on the shoulder: the last test date and the next one due. The interval depends on the country and the type of bottle.
• The valve: it should open smoothly, with clean threads and no oil or grease anywhere near it.
• Whether it has a siphon or dip tube. A siphon bottle delivers liquid carbon dioxide and will frost and ruin the regulator, which wants gas from the top of the cylinder.
• Ownership markings. Some bottles belong to the gas company for life and nobody else is allowed to refill them.
• The outside: deep pitting, dents, a bent foot ring or a seized collar all mean the depot may refuse to fill it.
• What was last in it. If the seller cannot say, walk away rather than connect it to a tank.

What happens if the bottle fails its test?

A cylinder that fails is not scrap to you. Most depots will take it in exchange for a tested one, usually for the price of a fill plus a handling charge. Ask about that policy before you buy a bottle with an old stamp, because it sets the real price of the bargain.

Once the cylinder is home: connect it safely

Stand the bottle upright and secure it, strapped or boxed so it cannot be knocked over, away from heaters and direct sun. Close the regulator's needle valve, screw the regulator on with a fresh washer, then open the cylinder valve slowly and watch the working pressure settle before you touch the bubble count.

Then leak-check every joint with soapy water, and fit a precision check valve in the line so tank water cannot siphon back towards the regulator when the solenoid closes at night. Watch the bubble counter for an evening before you trust a setting, and let the drop checker in the tank, not the gauge on the bottle, tell you whether the level is right.

Write the fill date on a strip of tape on the cylinder. After the second refill you will know how long a bottle lasts on your tank, which is the number that decides what size you buy next time.

Frequently asked questions

Where can I buy a CO2 cylinder for an aquarium?

From welding and industrial gas suppliers, homebrew and beverage shops, fire-extinguisher and safety companies, and some aquascaping stores. Look for the ones that fill or exchange bottles in your own town, then ask which sizes they stock and whether the bottle would be yours or theirs.

Can I buy a CO2 cylinder online?

Empty cylinders are posted in some countries, but a filled one is dangerous goods and most couriers either refuse it or charge heavily. We ship regulators, diffusers and tubing, never the gas: the bottle is the part you collect and refill locally.

What size CO2 cylinder is best for a planted aquarium?

The largest one you can lift, store upright and get refilled conveniently. A bigger bottle costs less per fill and runs for months between swaps, while a smaller one hides in a cabinet. The sizes on sale differ by market, so ask your local filler what they refill.

Do I need a special regulator for my CO2 cylinder?

You need one built for your cylinder's valve thread. Ours come as CGA320 for North and Latin America, and BS341 / DIN477 / W21.8x14 for the UK, Europe, Japan and Taiwan. SodaStream, paintball and cartridge bottles use our adapter instead.

Is a SodaStream bottle enough for a planted tank?

With an adapter it works on a nano tank, or as cover while your main cylinder is away for a refill. On a larger planted tank it empties fast and the gas costs the most per fill of any route, so treat it as a bridge rather than the plan.

Once the bottle is sorted locally, the rest of the line is the easy part: washers, CO2-resistant tubing, check valves and drop checkers are all in our CO2 accessories.

Sources

• EIGA (European Industrial Gases Association) — guidance on the handling, storage and transport of gas cylinders
• HSE (UK) — Pressure systems: safety and you (INDG261) and the pressure systems guidance pages
• A national industrial gas supplier's cylinder catalogue (BOC, Linde or Air Liquide in the target country)
  
• ISO — cylinder marking and periodic inspection standards (the EN ISO 11119 and EN 1964 families)