W21.8 Bs341 Co2 Regulator Uk Fire Extinguisher
Direct answer: if your planted-tank CO2 source is a typical UK refillable CO2 cylinder or converted fire extinguisher, the connection you are most likely checking is BS341 No.8, DIN 477 No.6 or W21.8 x 14. Do not order on that assumption alone: confirm the cylinder valve marking first, because CGA320, M10 disposable bottles and proprietary aquarium cartridges need different fittings.
This matters because the regulator is the pressure-control point between a high-pressure cylinder and the aquarium. A nearly-right nut can feel close, then leak, cross-thread or require an adapter stack that makes the whole setup harder to check.
What the UK thread names mean
For UK aquascapers, the confusing part is that the same practical connection can appear under several names. CO2Art's UK cylinder guide describes BS341, DIN477 and W21.8 x 14 as the common refillable-cylinder family, while noting that CGA320 is a North American standard and M10 x 1 appears on some disposable welding bottles: CO2Art's guide to cylinder connections.
Aqua Essentials also frames fire extinguisher and pub-style refillable CO2 bottles as a common planted-tank route in the UK: Aqua Essentials on CO2 regulators for fire extinguishers.
Quick decision table
| Cylinder you have | Likely connection to verify | What to do before buying |
|---|---|---|
| UK refillable CO2 bottle or fire extinguisher | BS341 No.8 / DIN477 No.6 / W21.8 x 14 | Photograph the valve stamp and choose a native W21.8/BS341 regulator nut when available. |
| Imported North American bottle or regulator | CGA320 | Do not assume it fits a UK bottle. Check whether you need a verified adapter or a different regulator line. |
| Disposable welding CO2 bottle | Often M10 x 1 | Use the disposable-bottle connection only if your regulator is explicitly built for it. |
| Aquarium cartridge or SodaStream-style cylinder | Proprietary or adapter-specific | Check the exact adapter system rather than buying a standard fire-extinguisher regulator. |
When a native W21.8 regulator is the cleaner choice
If your cylinder is confirmed as UK refillable W21.8/BS341, a native matching regulator is usually simpler than stacking adapters. Adapter joints are not automatically unsafe, but every extra joint is another place to leak-test and another part that can be installed incorrectly.
UK retailers also sell aquarium regulators with DIN477 / BS341 No.8 / W21.8-14 compatibility stated directly on the product page, which is a useful benchmark for the wording you should look for: CO2 Supermarket's regulator connection wording.
Do not buy yet if the thread is unknown
Pause if the listing only says "CO2 bottle", if the valve stamp is hidden, or if you are borrowing a fire extinguisher and cannot confirm whether it is suitable for aquarium use. The human-friendly version: know the bottle mouth before you buy the tap.
Next step
If you have confirmed a refillable UK W21.8/BS341 cylinder and want a regulator line that includes a W21.8 option, check the same-site product details and select only the thread variant that matches your cylinder: DoublGauge Pro CO2 regulator details.
FAQ
Can I adapt CGA320 to W21.8?
Sometimes, with the correct rated adapter, but it adds a joint and should be leak-tested carefully. A native matching regulator is usually easier for a first setup.
Is every UK fire extinguisher suitable?
No. This article only helps you identify regulator thread fit. It does not certify the cylinder, valve condition, fill status or safe reuse.