Expensive CO2 for fish tank – solution how to make CO2 cheaper.

*** Disclaimer – everything described below you do on your own risk. ***

I do not have to convince anybody that the CO2 bubbles in aquarium really improves the plant growth. Unfortunately buying and then especially exchanging CO2 gas in dedicated aquarium cylinders is not very cheap especially when you need quite a lot of the CO2 in your tank(s). So to save some money my first idea was to use a CO2 fire extinguisher – I paid £25 for 2kg cylinder. Fine. My plan was to see any local CO2 extinguisher supplier and pay around £8 for the cylinder exchange. I described my CO2 installation HERE. I have heard that many people in some other parts of the UK get this swap for around £8 / 2kg CO2. Unfortunately I do not know anybody locally (Portsmouth/Havant area) who managed to get such a good price for the CO2. I found once company called Southern Fire Protection – based in Portsmouth. I do not recommend them! I called the company on Monday and asked if I get the cylinder replaced – they were OK so I appeared on Tuesday. I spoke to the technician and he told me the exchange will cost me £20. I was a bit surprised about the price but I decided to go ahead to save the planet and reuse the cylinder. When I wanted to pay whey told me that they accept only cash (what?!?!?!?!?!?!?!?!?) so I had to go home, as I was very busy that day, and I decided to come back next day – then the technician was not available and of course the lady in the office was unable to exchange my CO2 cylinder. They wasted a lot of my time plus petrol. Of course they ended up with 1 star review on google 🙂 The reason of describing the problem is to show that exchanging a CO2 fire extinguisher is not the easiest and cheaper way of getting CO2. I ended up with buying another CO2 extinguisher from Amazon. I was definitely not very happy bunny. And it is not the cheapest way either…

Few months before I decided to start using CO2 in my tank I saw one guy who was using a soda stream CO2 cylinder – which is very small – and it contains only 425g CO2. But I knew that it was quite expensive way the cylinder with CO2 gas costs £26.99 at Argos:

So it is the most expensive solution. But… Also I found that my aquarium stand is very limited with the height – see te picture below! I just had to drill a massive hole in the base to fit the cylinder 🙂

So I decided to go a bit different way. I bought a soda stream cylinder for £26.99. Is it expensive? For one off – no, to refill yes. But… Let’s start the solution for the cheap CO2 cylinder right now.

As mentioned – I bought the soda stream cylinder. Then I bought a TR21-4 to W21.8-14 adaptor. TR21-4 is a standard soda stream bottle cylinder thread, W21.8-14 is a standard thread for the CO2 gases in the UK and almost all Europe. Then you should see a CO2 regulator attached straight to the TR21-74 / W21.8-14 adaptor, but… I did mistake and I bought a wrong CO2 regulator with 5/8 BSP thread. So of course I had to buy another adaptor from W21.8-14 to 5/8 so for your set up it should be with one less element. Also, just for your curiosity – 2p coin helps me to adjust the bubbles more precisely as the standard knob diameter is very small.

So – Am I going to pay £10 each time I want to get 425g CO2? No. I am not crazy. I found company called Adamsgas – https://www.adamsgas.co.uk . They officially sell and refill CO2 gas. Sample prices are here:

Of course I had to pay £75.64 + VAT deposit for the bottle plus additional £20.64 + VAT for the 6.35kg (!!!) CO2 but it is very good price for the CO2 per kg! And of course I can return the cylinder any time I wish. And… they have hundreds of stockists around the UK – my is just 10 miles away from me in Hayling Island!

But… what can I do with such a massive cylinder? I can’t keep it at home as it is too big, also I worry a bit about what could happen if 6kg CO2 decompresses in one small room – I can only guess it could be fatal. So I keep the bottle outside, in my shed in the garden.

So now – how can I transfer the gas from the big cylinder into the small soda stream cylinder?


You can buy in eBay, Amazon or Aliexpress a special refill adapter hose. It is important to buy the right one! It should connect a soda stream bottle which has TR21-4 thread to the W21.8-14 thread which is in my gas large cylinder (and also all CO2 gas extinguishers). The W21.8-14 is also called DIN477 No. 6 and also it is called BS341 No. 8 or W0.860”-14TPI.

Do ***not*** buy CGA320 which is also called G1/2-14, it is used outside of Europe.

Transferring the gas from the big into the small cylinder is very easy and safe. As the compressed CO2 gas is liquid, it is impossible to transfer too much gas from the big cylinder into the small one. So it is safe and nothing will explode 🙂 But, it is quite important to put the soda stream empty cylinder into the freezer for about 2h, I forgot and I was able to refill only 220g CO2 instead of 425g. Next time I will be better! I have just found out that the CO2 gas, when the temperature reaches 31C CO2 becomes supercritical and fills the cylinder like a gas with the liquid density: LINK TO WIKI.

The refilling procedure is shown below:

Sheet 2 of the blue manual as shown above, says about flipping over the gas cylinder. My hand sketch below will explain why. The compressed CO2 is a liquid. The CO2 liquid sits at the bottom of the cylinder. Above is the gas. It is important to transfer a liquid CO2 from large to small cylinder. If you transfer the gas only, your taking gas cylinder will be almost empty. So my first transfer was done with the CO2 fire extinguisher. They always come with the syphon (standpipe valve) installed, as in case of fire it requires to release liquid CO2, which rapidly decompressed will become “frozen” – this is why the CO2 extinguishers are sometimes called snow extinguishers. The food grade CO2 usually comes in cylinders without syphon. To get the gas transferred from them you need to flip over the cylinder. Otherwise you won;t get the liquid withdrawal, you will get then the vapour withdrawal and we do not want that.

So FINALLY now my fish tank cabinet looks much better – I have got a lot of room for the CO2 gas cylinder!!! And now the CO2 gas is very cheap. But this is not everything. Scroll down a bit 🙂

To save the planet from getting more and more plastic PET bottles I also bought the soda stream machine and bow I make my own fizzy drinks:

So now a bit of the CO2 is for my aquarium, and a bit of CO2 is for my fizzy drinks 🙂 The good news is the another CO2 cylinder was included together with the soda stream machine so to sort everythign our I just had to buy one extra cylinder.

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