Just to add to this. I have a fluval edge 46l and I purchased a fluval CO2 kit which cost about 60 quid. It was a total waste of money and even for that sized tank I was buying a new cartridge every three weeks at a cost of 12 pound. I invested in a fire extinguisher and solenoid from co2 Art and this cost me around 80 quid. This was excluding the co2 resistant airline, diffuser and co2 checker, all of which I purchased on eBay. I'd say the total cost was about a 100 pound. I know it may seem a lot but getting the fluval co2 kit is a false economy as I haven't had to fill up my co2 extinguisher and don't expect to for at least six months and three bubbles a second rubbing all day an turned off at night. Plus the fluval ones are a pain having to keep turning them on and off manually.